Effect of Sodium Hypochlorite and Benzalkonium Chloride on the Structural Parameters of the Biofilms Formed by Ten Salmonella enterica Serotypes

Abstract

The influence of the strain on the ability of Salmonella enterica to form biofilms on polystyrene was investigated by confocal laser scanning microscopy. The effects of sodium hypochlorite with 10% active chlorine (SHY; 25,000, 50,000, or 100,000 ppm), and benzalkonium chloride (BZK; 1000, 5000, or 10,000 ppm) on twenty-four-hour-old biofilms was also determined. The biofilms of ten Salmonella enterica isolates from poultry (S. Agona, S. Anatum, S. Enteritidis, S. Hadar, S. Infantis, S. Kentucky, S. Thompson, S. Typhimurium, monophasic variant of S. Typhimurium 1,4,(5),12:i:-, and S. Virchow) were studied. Biofilms produced by S. Anatum, S. Hadar, S. Kentucky, and S. Typhimurium showed a trend to have the largest biovolume and the greatest surface coverage and thickness. The smallest biofilms (P < 0.01) in the observation field (14.2 × 10³ µm²) were produced by S. Enteritidis and S. 1,4,(5),12:i:- (average 12.9 × 10³ ± 9.3 × 10³ µm³) compared to the rest of the serotypes (44.4 × 10³ ± 24.7 × 10³ µm³). Biovolume and surface coverage decreased after exposure for ten minutes to SHY at 50,000 or 100,000 ppm and to BZK at 5000 or 10,000 ppm. However, the lowest concentrations of disinfectants increased biovolume and surface coverage in biofilms of several strains (markedly so in the case of BZK). The results from this study suggest that the use of biocides at low concentrations could represent a public health risk. Further research studies under practical field conditions should be appropriate to confirm these findings.

1. Introduction

Salmonellosis is the zoonosis associated with the largest number of outbreaks of food-borne disease in the European Union. It comes in second place after campylobacteriosis in terms of numbers of sporadic cases. Specifically, in 2017, Salmonella enterica was responsible for 1241 outbreaks of disease in the E.U. (with a total of 9600 associated instances). This was a figure that amounted to 24.4% of the 5079 outbreaks of food-borne disease, with products of animal origin being those most often involved in the infection. The number of confirmed cases of human salmonellosis in 2017 came to 91,662 (the notification rate was 19.7 cases per 100,000 inhabitants). The rate of admissions to hospital (calculated by using the number of cases of known status) was 42.5%, and the mortality rate was 0.25% [1]. Similarly, this is the pathogenic microorganism involved in the largest number of notifications passed to the Rapid Alert System for Food and Feed of the E.U. (RASFF). In 2017, Member States submitted 207 notifications relating to Salmonella out of a total of 414 notifications covering all pathogenic microorganisms. Third Countries submitted 471 notifications in respect of this microorganism out of a total of 561 notifications relating to pathogenic microorganisms [2].

The main serotypes of Salmonella implicated in human infections in the E.U. are Salmonella enterica serotype Enteritidis and Salmonella enterica serotype Typhimurium. In 2017 the first of these was involved in 49.1% and the second in 13.4% of cases of illness [1]. Other serotypes that are frequently concerned in human salmonellosis include Salmonella enterica serotype Agona, Salmonella enterica serotype Derby, Salmonella enterica serotype Hadar, Salmonella enterica serotype Infantis, Salmonella enterica serotype Kentucky, Salmonella enterica serotype Newport, Salmonella enterica serotype 1.4.[5].12:i:- (monophasic variant of S. Typhimurium), Salmonella enterica serotype Thompson, and Salmonella enterica serotype Virchow [1,3].

Salmonella enterica has the ability to form biofilms. These are complex microbial communities set in an extracellular polymer matrix produced by the bacteria themselves and adhering firmly to inert surfaces and living tissue, constituting the predominant form of microbial growth in nature. Cells in biofilms have a considerably increased resistance to environmental challenges, including disinfectants and antibiotics, relative to planktonic cells. This is the reason that biofilms have been identified as a major factor in the persistence of food-borne pathogens in food-processing environments [4]. Biofilms have been shown to be the main source for the contamination of foodstuffs and have been associated with more than 60% of outbreaks of food-borne illness, thus having become a significant problem in the food industry [5]. In addition, serious engineering problems also arise because of the presence of biofilms on the equipment and in the installations of food-processing plants.

Several compounds are commonly used as antimicrobials for sanitizing surfaces that come into contact with food [6]. Chlorine-based disinfectants, such as sodium hypochlorite (SHY), are oxidizing compounds that show a broad-spectrum bactericidal activity combined with high efficacy and low cost [7]. SHY is approved for use in the European Economic Area (EEA) and/or Switzerland as disinfectant for food and feed areas (product-type 4) [8]. According to Commission Implementing Regulation (EU) 2017/1273, active chlorine released from sodium hypochlorite is approved as an active substance for use in biocidal product-types 1, 2, 3, 4, and 5 when active chlorine concentration in aqueous solution is ≤ 1800 ppm (i.e., ≤ 18% w/w). Quaternary ammonium compounds, such as benzalkonium chloride (BZK), are cationic surfactants that act by disrupting lipid membrane bilayers, being effective against a number of pathogenic microorganisms [9]. BZK is being reviewed for use as a biocide in the EEA area and/or Switzerland for food and feed area disinfection [10]. For both disinfectants to be effective, they must be used at appropriate concentrations.

Prior studies have noted major differences between serotypes with regard to their capacity to form a biofilm [11,12]. The effect of several disinfectants on Salmonella biofilms has been also assessed [13,14]. Nonetheless, viable counts, crystal violet assay, or scanning electron microscopy are used to study biofilms in most reports, and it would seem that research works quantitatively assessing the effect of different concentrations of SHY and BZK on structural parameters (e.g., biovolume, surface coverage, thickness, or roughness) of biofilms formed by different Salmonella serotypes are lacking. Thus, there is a need for further research that would allow quantitative comparisons of the three-dimensional structures and the structural parameters of biofilms of various serotypes of Salmonella before and after exposure to different concentrations of several disinfectants. The aim of this research work was to compare, by means of confocal laser scanning microscopy (CLSM) and quantitative image analysis, the structural parameters of biofilms formed by strains belonging to ten different serotypes of Salmonella and to determine the effectiveness of various concentrations of two food-grade biocides (sodium hypochlorite and benzalkonium chloride) on these structures. The ultimate objective was to identify the concentrations that were fittest for this purpose.

2. Results and Discussion

2.1. Architecture of Salmonella enterica Biofilms

All strains of S. enterica formed biofilm on polystyrene microtiter plates after 24 h of incubation at 37 °C. The untreated control biofilms presented values varying between serotypes in respect of biovolume in the observation field of 14.2 × 10³ µm² (running from 8.2 × 10³ ± 1.3 × 10³ to 76.5 × 10³ ± 34.1 × 10³ µm³; Table 1), percentage of surface coverage (from 29.1 ± 2.8% to 96.9 ± 3.8%; Table 2), maximum thickness (between 15.7 ± 2.1 and 53.3 ± 13.8 µm; Table 3), and roughness (going from 0.442 ± 0.060 to 0.682 ± 0.048; Table 4). The serotypes S. Kentucky and S. Hadar showed a trend to produce biofilms with the greatest biovolume, with values in the field observed (14.2 × 10³ µm²) of 76.5 × 10³ ± 34.1 × 10³ and 71.8 × 10³ ± 24.8 × 10³ µm³, respectively. These were followed (P > 0.05) by S. Anatum (54.0 × 10³ ± 11.7 × 10³ µm³) and S. Typhimurium (47.5 × 10³ ± 6.3 × 10³ µm³). Biofilms of these four serotypes also showed a trend to have the highest percentage of surface coverage (between 88.9 ± 6.2% and 96.9 ± 3.8%) and the greatest thickness (running from 34.7 ± 4.5 to 53.3 ± 13.8 µm). S. Enteritidis and S. Typhimurium monophasic variant 1,4,(5),12:i:- showed lower figures (P < 0.01) for biovolume, percentage of surface covered, and maximum thickness (12.9 × 10³ ± 9.3 × 10³ µm³, 32.4 ± 6.8%, and 18.3 ± 4.3 µm, respectively, as average) than the rest of the serotypes (44.4 × 10³ ± 24.7 × 10³ µm³, 83.2 ± 12.1%, and 34.6 ± 12.2 µm, respectively). S. Enteritidis and S. 1,4,(5),12:i:-, unlike the others serotypes, formed only microcolonies of non-confluent cells after twenty-four hours of incubation at 37 °C (Figure 1).

Table 1.

Biovolume (thousands of µm³ in the observation field of 14.2 × 10³ µm²) of twenty-four-hour-old biofilms of several Salmonella strains of food origin after exposure to different concentrations of sodium hypochlorite (SHY; 10% of active chlorine).

SEROTYPE	Strains Exposed to:
	Nonexposed	SHY-25,000 ppm	SHY-50,000 ppm	SHY-100,000 ppm
Salmonella Agona	23.6 ± 4.6abab	52.2 ± 21.5bbc	14.2 ± 4.3aa	16.5 ± 7.6aa
Salmonella Anatum	54.0 ± 11.7bbcd	49.8 ± 2.6bbc	24.4 ± 6.1aab	18.2 ± 5.4aa
Salmonella Enteritidis	17.5 ± 12.2aab	47.6 ± 16.1babc	13.9 ± 9.2aa	2.4 ± 0.3aa
Salmonella Hadar	71.8 ± 24.8bcd	65.9 ± 21.8bc	16.3 ± 7.1aab	15.4 ± 6.9aa
Salmonella Infantis	31.7 ± 14.1babc	15.8 ± 1.8aba	11.8 ± 3.7aa	3.1 ± 1.1aa
Salmonella Kentucky	76.5 ± 34.1bd	30.9 ± 4.7abab	15.8 ± 4.9aa	11.8 ± 9.6aa
Salmonella Thompson	27.9 ± 1.5babc	31.5 ± 3.6bab	25.8 ± 3.9abab	16.9 ± 6.3aa
Salmonella Typhimurium	47.5 ± 6.3babcd	45.8 ± 10.6babc	38.2 ±18.0abc	15.4 ± 6.9aa
Salmonella 1,4,(5),12:i-	8.2 ± 1.3ba	32.9 ± 2.8dabc	15.6 ± 1.0ca	3.9 ± 0.5aa
Salmonella Virchow	22.5 ± 1.0bab	23.1 ± 1.3bab	13.7 ± 3.0aa	11.1 ± 5.0aa

Data (mean ± SD; n = 9) in the same row with no letters in common (superscript) are significantly different (P < 0.05). Data in the same column with no letters in common (subscript) are significantly different (P < 0.05).

Table 2.

Percentage of surface coverage of twenty-four-hour-old biofilms of several Salmonella strains of food origin after exposure to different concentrations of sodium hypochlorite (SHY; 10% active chlorine).

SEROTYPE	Strains Exposed to:
	Nonexposed	SHY-25,000 ppm	SHY-50,000 ppm	SHY-100,000 ppm
Salmonella Agona	79.6 ± 6.3bccd	96.0 ± 3.4ccd	61.1 ± 11.6abab	39.2 ± 17.6abc
Salmonella Anatum	90.0 ± 5.2bcde	93.7 ± 3.1bcd	80.1 ± 5.8abb	68.5 ± 9.8ade
Salmonella Enteritidis	35.7 ± 8.8ba	95.1 ± 3.6dcd	55.5 ± 4.3ca	14.7 ± 3.3aa
Salmonella Hadar	88.9 ± 6.2ccde	98.6 ± 1.3cd	62.2 ± 3.0aab	78.0 ±3.2be
Salmonella Infantis	61.8 ± 8.9bb	61.8 ± 5.3ba	55.0± 12.1ba	12.8± 1.9aa
Salmonella Kentucky	96.9 ± 3.8ce	87.8 ± 3.5cc	64.7 ± 10.7bab	22.3 ± 7.1aab
Salmonella Thompson	75.7 ± 6.5bbc	87.4 ± 2.9bc	82.4 ± 4.3bb	53.7 ± 10.5acd
Salmonella Typhimurium	94,9 ± 0,4bde	95.3 ± 3.6bcd	73.2 ± 7.3aab	68.0 ± 8.8ade
Salmonella 1,4,(5),12:i-	29.1 ± 2.8ba	89.4 ± 2.3dcd	66.5 ± 2.7cab	21.8 ± 1.7aab
Salmonella Virchow	77.5 ± 1.3cbc	74.3 ± 1.2bcb	60.5 ± 7.1aab	64.6 ± 3.0abde

For interpretation, see Table 1.

Table 3.

Maximum thickness (µm) of twenty-four-hour-old biofilms of several Salmonella strains of food origin after exposure to different concentrations of sodium hypochlorite (SHY; 10% active chlorine).

SEROTYPE	Strains Exposed to:
	Nonexposed	SHY-25,000 ppm	SHY-50,000 ppm	SHY-100,000 ppm
Salmonella Agona	33.3 ± 14.6aabc	26.0 ± 5.3aa	16.3 ± 4.5aa	24.7 ± 8.1aabc
Salmonella Anatum	34.7 ± 4.5aabc	32.3 ± 0.6aa	30.0 ± 11.5aa	26.0 ± 8.7aabc
Salmonella Enteritidis	21.0 ± 4.6aab	38.7 ± 17.8aa	20.7 ± 14.4aa	12.3 ± 3.8aa
Salmonella Hadar	42.3 ± 2.5abc	30.0 ± 15.6aa	27.0 ± 6.2aa	41.7 ± 13.7ac
Salmonella Infantis	25.3 ± 7.5aab	29.3 ± 2.1aa	17.3 ± 4.6aa	19.7 ± 4.5aabc
Salmonella Kentucky	35.7 ± 9.2aabc	29.7 ± 4.7aa	27.0 ± 11.1aa	25.7 ± 0.6aabc
Salmonella Thompson	22.0 ± 2.7aab	51.3 ± 4.7ba	33.7 ± 16.6aba	38.0 ± 10.0abbc
Salmonella Typhimurium	53.3 ± 13.8ac	29.3 ± 8.5aa	33.0 ± 2.7aa	41.7 ± 13.7ac
Salmonella 1,4,(5),12:i-	15.7 ± 2.1aa	27.7 ± 5.9aa	17.7 ± 4.2aa	15.7 ± 6.7aab
Salmonella Virchow	30.0 ± 10.2aabc	33.7 ± 8.6aa	18.7 ± 5.7aa	15.3 ± 6.1aab

For interpretation, see Table 1.

Table 4.

Roughness of twenty-four-hour-old biofilms of several Salmonella strains of food origin after exposure to different concentrations of sodium hypochlorite (SHY; 10% active chlorine).

SEROTYPE	Strains Exposed to:
	Nonexposed	SHY-25,000 ppm	SHY-50,000 ppm	SHY-100,000 ppm
Salmonella Agona	0.451 ±0.019aab	0.400 ± 0.034aab	0.430 ± 0.044aa	0.673 ± 0.119bbc
Salmonella Anatum	0.586 ± 0.068babcd	0.451 ± 0.003abcd	0.450 ± 0.047aa	0.464 ± 0.038aabc
Salmonella Enteritidis	0.682 ± 0.048bd	0.414 ± 0.025aabc	0.430 ± 0.112aba	0.258 ± 0.151aa
Salmonella Hadar	0.644 ± 0.067bd	0.358 ± 0.035aa	0.428 ± 0.017aa	0.516 ± 0.123ababc
Salmonella Infantis	0.628 ± 0.031bcd	0.511 ± 0.019abde	0.408 ± 0.056aa	0.684 ± 0.124bbc
Salmonella Kentucky	0.442 ± 0.060aa	0.450 ± 0.019abcd	0.437 ± 0.017aa	0.707 ± 0.024bc
Salmonella Thompson	0.567 ± 0.089aabcd	0.476 ± 0.015acde	0.448 ± 0.033aa	0.514 ± 0.149aabc
Salmonella Typhimurium	0.474 ± 0.071ababc	0.411 ± 0.024aabc	0.530 ± 0.023ba	0.453 ± 0.007ababc
Salmonella 1,4,(5),12:i-	0.609 ± 0.035bbcd	0.462 ± 0.024abcd	0.439 ± 0.008aa	0.340 ± 0.096aa
Salmonella Virchow	0.452 ± 0.031aab	0.550 ± 0.039be	0.434 ± 0.031aa	0.410 ± 0.030aab

For interpretation, see Table 1.

Figure 1.

Three-dimensional projections of biofilms formed by ten strains of Salmonella enterica before and after exposure to sodium hypochlorite (SHY) at various concentrations. These were obtained from optical sections one micron in size along the z axis using a confocal laser scanning microscope. Virtual projections of the shadow to the right are shown, with each square indicated representing a side of 119 μm in length.

The fact that all strains of S. enterica studied had the ability to produce biofilm on polystyrene surfaces is a cause for concern in the context of food safety and public health. This is because the plastic in question is material in very wide use on cattle farms, in slaughterhouses, in food-processing plants, and in establishments serving food. It is frequently used to manufacture a range of surfaces such as piping, cutting boards, and other equipment [15,16]. Moreover, a number of researchers have demonstrated that there is a positive correlation between the production of biofilm on polystyrene microtiter plates and the formation of biofilms on various materials used for surfaces in the food industry [17,18].

The formation of biofilms is one of the commonest strategies used by bacteria in tolerating various sorts of environmental stress [19]. Biofilms increase microbial resistance to physical, biological and chemical agents (for example, antimicrobials), so that the capacity to form biofilms contributes greatly to the persistence of bacteria in food-processing installations [18,20]. Indeed, molecular techniques have been used to show that certain strains of Salmonella can remain for several years in food-processing plants [21]. Furthermore, biofilms that are formed in food-processing environments pose a major problem for the food industry, as these structures have been identified as the principal source of contamination of foodstuffs with pathogenic and spoilage microorganisms. This fact constitutes a challenge for public health and involves considerable financial losses [22]. For instance, on these lines it has been shown that the percentage of poultry carcasses contaminated with Salmonella grows significantly during processing, as a consequence of the presence of bacteria on surfaces with which these foods come into contact [23].

The present study found striking differences between serotypes with regard to their capacity to form biofilm, as had also been observed previously [11,12,18]. S. Kentucky and S. Hadar formed strong biofilms (high values for biovolume, percentage of surface covered, and maximum thickness). The fact that S. Kentucky is a powerful producer of biofilm is particularly noteworthy, as this serotype is characterized by normally having a high level of resistance to antibiotics of clinical importance, such as ciprofloxacin [24]. The results of the research being reported here do not coincide completely with the findings from previous investigations, in which it was observed that Salmonella Hadar (strain SH174) had only a slight ability to form biofilm on polystyrene after 24 h of incubation (15.3 × 10³ ± 4.7 × 10³ µm³ in the observation field of 14.2 × 10³ µm²) [25]. Similarly, it is not in agreement with the observations of other researchers indicating that S. Kentucky and S. Hadar are moderate producers of biofilm [26]. These discrepancies among research may be due to variations between different strains of the same serotype with regard to their abilities to form biofilm [11].

The strain of S. Typhimurium tested was a strong producer of biofilm. The considerable capacity of strains of this serotype to form biofilm on polystyrene has also been highlighted in previous work [27] in which, after 24 h of incubation on glass at 37 °C, biofilms with a biovolume of 129.4 × 10³ ± 34.7 × 10³ µm³ were seen in the field of observation (14.2 × 10³ µm²) for another strain of S. Typhimurium (strain S175). The great ability to form biofilm that was seen in S. Typhimurium in the present study is a finding of considerable interest from the viewpoint of public health, as this serotype is among the most dangerous to humans. Sarwari et al. [28] developed a mathematical model for predicting the capacity to cause human illness, and Salmonella Typhimurium attained the highest score of the seven serotypes compared [29]. On the other hand, some authors have noted that the strains of this serotype produce little biofilm on microtiter plates and do so slowly [18].

Several researchers have demonstrated that strains of Salmonella Agona have a substantial capacity to form biofilm [11,30]. However, the strain of S. Agona trialed in the present work was among those producing the least biofilm. Finally, strains of the serotypes S. Infantis, S. Virchow, and S. Enteritidis are, in general, poor at producing biofilm [31], this fact providing backing for some of the findings of the study being reported here.

2.2. Effect of Differing Concentrations of SHY and BZK on Biofilms of Salmonella enterica

Biofilms of Salmonella formed on polystyrene after 24 h of incubation at 37 °C were exposed for 10 min to aqueous solutions of SHY (25,000, 50,000, or 100,000 ppm) or BZK (1000, 5000, or 10,000 ppm).

Use of SHY at 25,000 ppm (2500 ppm of active chlorine) did not reduce the biovolume or the surface coverage of the biofilms in any case. Indeed, for some strains (S. Enteritidis and S. 1,4,(5),12:i-) this treatment was even associated with an increase in biovolume and in the percentage of surface coverage of the biofilm (Table 1 and Table 2, Figure 1). Even though additional research studies are needed to support these findings, our results suggest that the concentrations of SHY habitually used in disinfecting equipment and installations (800 to 2000 ppm of active chlorine) [9,32] could fail to be effective in eliminating biofilms of Salmonella when the disinfectant is applied for a ten-minute period. On these lines, Holah [33] indicated that exposure times in excess of ten minutes are advisable when disinfecting utensils or equipment, especially when they are hard for disinfectants to reach. Furthermore, it should be noted that there are circumstances in which disinfectants are applied in small doses, for instance as the consequence of an inaccurate calculation of concentrations, inappropriate storage of the chemicals, the difficulty of reaching certain locations, or the presence of excessive quantities of organic matter, which reduces the effectiveness of some biocides, such as chlorine compounds [34]. When SHY was applied at 50,000 or 100,000 ppm (5000 or 10,000 ppm of active chlorine, respectively), reductions in biovolume and in the percentage of surface covered by the biofilm were observed relative to the control samples in the majority of cases (Table 1 and Table 2, Figure 1).

One noteworthy feature of this work is the increase in biomass observed after exposure to BZK at 1000 ppm. This treatment caused a significant growth in the biovolume of films relative to unexposed strains in S. Agona, S. Enteritidis, S. Thompson, and S. 1,4,(5),12:i:- (Table 5, Figure 2). BZK at 1000 ppm brought about an increase (P < 0.05) in percentage of surface covered in S. Agona, S. Enteritidis, S. Infantis, S. Thompson, and S. 1,4,(5),12:i:- (Table 6, Figure 2). This treatment also caused a marked growth in the maximum thickness of the biofilm in the case of S. Agona, S. Enteritidis, S. Infantis, and S. 1,4,(5),12:i-, while the maximum thickness of the film diminished in strains of the serotypes S. Anatum and S. Typhimurium (Table 7, Figure 2).

Table 5.

Biovolume (thousands of µm³ in the observation field of 14.2 × 10³ µm²) of twenty-four-hour-old biofilms of several Salmonella strains of food origin after exposure to different concentrations of benzalkonium chloride (BZK).

SEROTYPE	Strains Exposed to:
	Nonexposed	BZK-1000 ppm	BZK-5000 ppm	BZK-10,000 ppm
Salmonella Agona	23.6 ± 4.6aab	177.8 ± 50.5bd	10.2 ± 1.1aa	8.2 ± 7.4aab
Salmonella Anatum	54.0 ±11.7bbcd	24.5 ± 9.7aa	10.2 ± 1.5aa	11.8 ± 6.5aab
Salmonella Enteritidis	17.5 ± 12.2aab	121.9 ± 58.3bbc	13.9 ± 1.7aabc	4.7 ± 4.2aab
Salmonella Hadar	71.8 ± 24.8ccd	57.3 ± 4.6bcab	30.9 ± 4.4abd	10.3 ± 0.7aab
Salmonella Infantis	31.7 ± 14.1ababc	51.6 ± 19.1bab	10.5 ± 4.2aab	2.9 ± 1.2aa
Salmonella Kentucky	76.5 ± 34.1bd	19.8 ± 3.8aa	11.3 ± 4.0aab	9.9 ± 1.8aab
Salmonella Thompson	27.9 ± 1.5babc	63.0 ± 7.3cab	17.2 ± 3.4ababcd	14.2 ± 1.5ab
Salmonella Typhimurium	47.5 ± 6.3babcd	14.2 ± 8.8aa	21.1 ± 3.9aabcd	14.1 ± 1.3ab
Salmonella 1,4,(5),12:i-	8.2 ± 1.3aa	125.7 ± 50.1bbc	26.9 ± 13.2acd	4.1 ± 0.8aab
Salmonella Virchow	22.5 ± 1.0bcab	16.5 ± 3.4ba	25.4 ± 3.2cbcd	8.0 ± 1.0aab

For interpretation, see Table 1.

Figure 2.

Three-dimensional projections of biofilms formed by ten strains of Salmonella enterica before and after exposure to benzalkonium chloride (BZK) at various concentrations. These were obtained from optical sections one micron in size along the z axis using a confocal laser scanning microscope. Virtual projections of the shadow to the right are shown, with each square indicated representing a side of 119 μm in length.

Table 6.

Percentage of surface coverage of twenty-four-hour-old biofilms of several Salmonella strains of food origin after exposure to different concentrations of benzalkonium chloride (BZK).

SEROTYPE	Strains Exposed to:
	Nonexposed	BZK-1000 ppm	BZK-5000 ppm	BZK-10,000 ppm
Salmonella Agona	79.6 ± 6.3ccd	100.0 ± 0.0de	32.6 ± 4.0bb	9.8 ± 3.7aab
Salmonella Anatum	90.0 ± 5.2ccde	78.7± 3.5cc	50.3 ± 3.7bc	21.8 ± 3.9abcd
Salmonella Enteritidis	35.7 ± 8.8ba	99.5 ± 0.9cde	31.8 ± 1.4bab	6.0 ± 3.3aa
Salmonella Hadar	88.9 ± 6.2ccde	98.0 ± 0.6cde	50.7 ± 4.7bc	33.4 ± 1.0ad
Salmonella Infantis	61.8 ± 8.9bb	90.0 ± 4.1cd	20.6 ± 5.1aa	9.4 ± 1.2aab
Salmonella Kentucky	96.9 ± 3.8de	70.7 ± 6.2cc	52.2 ± 3.2bc	26.9 ± 1.4acd
Salmonella Thompson	75.7 ± 6.5cbc	98.4 ± 0.7dde	36.5 ± 1.8bb	25.5 ± 2.1acd
Salmonella Typhimurium	94.9 ± 0.4bde	48.1 ± 4.7aa	36.2 ± 5.5ab	35.9 ± 11.2ad
Salmonella 1,4,(5),12:i-	29.1 ± 2.8ba	99.6 ± 0.6dde	78.6 ± 5.2cd	13.5 ± 3.4aabc
Salmonella Virchow	77.5 ± 1.3dbc	60.6 ± 4.8cb	34.0 ± 3.7bb	21.0 ± 2.0aabcd

For interpretation, see Table 1.

Table 7.

Maximum thickness (µm) of twenty-four-hour-old biofilms of several Salmonella strains of food origin after exposure to different concentrations of benzalkonium chloride (BZK).

SEROTYPE	Strains Exposed to:
	Nonexposed	BZK-1000 ppm	BZK-5000 ppm	BZK-10,000 ppm
Salmonella Agona	33.3 ± 14.6aabc	120.3 ± 44.6bb	23.7 ± 6.1aabcd	23.3 ± 9.1aa
Salmonella Anatum	34.7 ± 4.5cabc	19.3 ± 2.1ba	7.7 ± 0.6aa	22.3 ± 2.5ba
Salmonella Enteritidis	21.0 ± 4.6aab	107.0 ± 40.0bb	20.3 ± 2.1aabc	20.3 ± 12.3aa
Salmonella Hadar	42.3 ± 2.5bbc	31.0 ± 5.3aba	42.7 ± 11.2bd	22.7 ± 0.6aa
Salmonella Infantis	25.3 ± 7.5abab	41.3 ± 9.8ca	23.7 ± 5.7ababcd	22.3 ± 4.2aa
Salmonella Kentucky	35.7 ± 9.2babc	31.7 ± 4.0aba	17.7 ± 2.5aab	21.0 ± 2.7aa
Salmonella Thompson	22.0 ± 2.7aab	31.3 ± 9.3aa	23.0 ± 4.6aabc	20.3 ± 1.2aa
Salmonella Typhimurium	53.3 ± 13.8bc	16.7 ± 2.9aa	28.3 ± 2.9abcd	22.7 ± 3.2aa
Salmonella 1,4,(5),12:i-	15.7 ± 2.1aa	102.0 ± 14.8bb	38.3 ± 14.2acd	24.3 ± 0.6aa
Salmonella Virchow	30.0 ± 10.2aabc	28.7 ± 10.0aa	34.3 ± 2.1abcd	21.0 ± 1.7aa

For interpretation, see Table 1.

An increased ability to form biofilm in the presence of low doses of SHY or BZK has been demonstrated previously for strains of Escherichia coli [34], methicillin-resistant Staphylococcus aureus (MRSA) [6], Salmonella [27], and Listeria monocytogenes [35]. In the present study the biofilms were exposed to disinfectants for ten minutes, and the biocides were then eliminated, even though residual quantities probably remained in the wells of the microtiter plate. A period of two to four hours elapsed between treatment with biocides and microscopic observation, a period during which the exposed strains (in contact with residual amounts of biocides) were able to synthesize biofilm to a greater extent than unexposed strains. The explanation for this greater production of biofilm by the strains exposed to residual doses of SHY or BZK may have to do with the adaptational response of the bacteria, appearing as changes to the structure, composition, or speed of growth of the bacterial cells or as an increase in the extracellular polymer matrix. The part that may be played by certain specialized cellular structures (for example, fimbriae and pili) in the augmented capacity of bacteria to form biofilm has also been highlighted [6]. Nonetheless, further studies would be necessary to confirm these hypotheses.

BZK is usually employed at a dosage of 1000 to 5000 ppm [9,36,37]. The increase in the biomass of biofilms after exposure for ten minutes to BZK at 1000 ppm underlines the need to further research into practical applications (i.e., on surfaces and equipment present in food-processing facilities) to substantiate these findings and to determine whether the recommended concentrations of BZK should be revised. This is especially crucial in the case of Gram-negative bacteria, which normally are more resistant to quaternary ammonium products as a consequence of modifications in the permeability of their cell walls or through an increased expression of unspecific efflux pumps [38,39].

At concentrations of 5000 and 10,000 ppm, BZK was effective in reducing the biovolume of the biofilms formed by most of the strains of Salmonella (Table 5). Exposure to BZK at 5000 ppm reduced the percentage of surface covered relative to the control strains in all cases, with the exception of S. Enteritidis and S. 1,4,(5),12:i:-. At 10,000 ppm, BZK was efficacious in decreasing the percentage of surface covered by the biofilms of all the strains (Table 6). At 5000 and 10,000 ppm of BZK, the maximum thickness of the biofilms decreased in the case of S. Anatum, S. Hadar (10,000 ppm), S. Kentucky, and S. Typhimurium (Table 7). These observations coincide with those in studies undertaken by other researchers, who have noted that the biocides used in the food industry are variable in their effects, which go from virtually total elimination of Salmonella cells down to a very slight or almost zero reduction, depending on a series of factors, among which the concentrations at which they are used is prominent [40].

Considerable differences were observed in the behavior of the various serotypes in respect to the surface roughness, or rugosity, of the biofilms after exposure to the disinfectants tested. After treatment with SHY, surface roughness tended to decrease in comparison to the unexposed biofilms in the cases of S. Anatum, S. Enteritidis, S. Hadar, S. Infantis, and S. 1,4,(5),12:i:-. However, an increase in roughness was observed for biofilms of S. Virchow after treatment with 25,000 ppm of SHY and for biofilms of S. Agona and S. Kentucky after treatment with 100,000 ppm of SHY (Table 4). When the biofilms were exposed to 1000 ppm of BZK, rugosity of biofilms formed by S. Anatum, S. Enteritidis, S. Hadar, S. Thompson, and S. 1,4,(5),12:1- decreased. After treatment with BZK at 5000 ppm or 10,000 ppm, roughness increased in the cases of the serotypes S. Agona (10,000 ppm), S. Anatum (10,000 ppm), S. Kentucky (10,000 ppm), S. Typhimurium, and S. Virchow. BZK at 5000 ppm decreased roughness in S. Anatum and S. 1,4,(5),12:1- biofilms. In the other serotypes investigated, no variations were seen in the rugosity of biofilms (Table 8).

Table 8.

Roughness of twenty-four-hour-old biofilms of several Salmonella strains of food origin after exposure to different concentrations of benzalkonium chloride (BZK).

SEROTYPE	Strains Exposed to:
	Nonexposed	BZK-1000 ppm	BZK-5000 ppm	BZK-10,000 ppm
Salmonella Agona	0.451 ± 0.019abab	0.246 ± 0.028aa	0.668 ± 0.065bccd	0.702 ± 0.170ca
Salmonella Anatum	0.586 ± 0.068babcd	0.439 ± 0.006abcd	0.395 ± 0.031aa	0.703 ± 0.033ca
Salmonella Enteritidis	0.682 ± 0.048bd	0.335 ± 0.058aabc	0.684 ± 0.026be	0.648 ± 0.192ba
Salmonella Hadar	0.644 ± 0.067bd	0.397 ± 0.008aabcd	0.676 ± 0.051be	0.668 ± 0.028ba
Salmonella Infantis	0.628 ± 0.031acd	0.649 ± 0.142ae	0.617 ± 0.006acd	0.696 ± 0.202aa
Salmonella Kentucky	0.442 ± 0.060aa	0.503 ± 0.044acde	0.419 ± 0.051aa	0.699 ± 0.067ba
Salmonella Thompson	0.567 ± 0.089babcd	0.391 ± 0.005aabcd	0.563 ± 0.047bbc	0.563 ± 0.003ba
Salmonella Typhimurium	0.474 ± 0.071aabc	0.407 ± 0.020aabcd	0.626 ± 0.020bcd	0.701 ± 0.016ba
Salmonella 1,4,(5),12:i-	0.609 ± 0.035cbcd	0.314 ± 0.075aab	0.464 ± 0.027bab	0.696 ± 0.019ca
Salmonella Virchow	0.452 ± 0.031aab	0.553 ± 0.060abde	0.639 ± 0.022bccd	0.726 ± 0.033ca

For interpretation, see Table 1.

3. Materials and Methods

3.1. Salmonella Strains and Disinfectants

Ten strains of Salmonella enterica were used, derived from samples of chicken meat (S. Agona, S. Anatum, S. Enteritidis, S. Hadar, S. Infantis, S. Kentucky, S. Thompson, S. Typhimurium, S. 1,4,[5],12:i:-, and S. Virchow). The strains were stored frozen at −30 °C in tryptone soya broth (TSB, Oxoid Ltd., Hampshire, United Kingdom) with 20% glycerol (vol/vol). In order to carry out the research work, strains were transferred to tubes of TSB, incubated at 37 °C for twenty-four hours, and then inoculated by streaking on tryptone soya agar plates (TSA, Oxoid). Plates were incubated at 37 °C for twenty-four hours and thereafter stored at 4 °C while the experiments were performed. The trials used two disinfectants: sodium hypochlorite (SHY, with 10% of active chlorine; Sigma-Aldrich, Steinheim, Germany) at concentrations of 25,000, 50,000, and 100,000 ppm (2500, 5000, and 10,000 ppm of active chlorine, respectively) and benzalkonium chloride (BZK; Fluka, Deisenhofen, Germany) at concentrations of 1000, 5000, and 10,000 ppm. These concentrations were selected because it had been previously observed that doses below 2500 ppm of active chlorine and below 1000 ppm of BZK were ineffective to eliminate biofilms in the experimental conditions studied (data not shown). The concentrations tested are similar to or higher than the habitually used doses for application in food contact surfaces, which are 800 to 2000 ppm of active chlorine [9,32] and 1000 to 5000 ppm of BZK [9,36,37]. The solutions were prepared with sterile distilled water under aseptic conditions immediately before the experiments.

3.2. Biofilm Formation and Analysis

The strains were inoculated into tubes of TSB, which were incubated for 24 h at 37 °C to obtain cultures with approximately 10⁹ cfu/mL. Three decimal dilutions were then prepared in the same culture broth. Microtiter plates were used (Matrix 96-well polystyrene flat-bottom microplates; Thermo Fisher Scientific, Rochester, NY, USA). A volume of 250 µL (approximately 10⁶ cfu/mL) of the third dilution of the culture to be studied was added to the wells. The plates were incubated for one hour at 37 °C to permit bacterial adhesion. After this time had elapsed, the wells were rinsed with a solution of sodium chloride (150 mM NaCl; Sigma-Aldrich, Steinheim, Germany) so as to eliminate any nonadherent cells. The wells were then topped up with 250 µL of sterile TSB. The plate was incubated for 24 h at 37 °C to allow the development of biofilms. The next step was to rinse the wells once again with sodium chloride solution (150 mM NaCl) and add the various solutions of disinfectants, or sterile distilled water in the case of controls. In all cases, contact time was 10 min. The wells were then rinsed with 250 µL of NaCl 150 mM. For staining with fluorescent dye, a volume of 1.25 µL of SYTO 9 (Invitrogen, Barcelona, Spain) was added to 1000 µL of TSB, and 250 μL of this solution was put into each well. The plate was then incubated in the dark at 37 °C for 20 min to enable fluorescent labeling of the bacteria.

The following procedure was used to acquire images with the aid of a Nikon Eclipse TE 2000-U confocal scanning laser microscope using the EZ-C13.60 program (Nikon Instruments Inc., Melville, NY, USA). All biofilms were scanned at 400 Hz, using a 40× objective lens with a 488 nm argon laser set at 90% intensity. Three stacks of horizontal plane images (512 by 512 pixels, corresponding to an area of 119 by 119 µm) with a z step of 1 µm were acquired for each biofilm, using different areas in the well. Three independent experiments were performed for each condition, on different days. Thus, a total of 630 CLSI images were obtained: 10 strains × 7 treatments (2 chemicals × 3 concentrations + controls) × 3 stacks in each biofilm × 3 experiments.

In the processing of the images, use was made of a previously described method [34]. Three-dimensional images of the biofilms were reconstructed by means of the IMARIS 9.1 program (Bitplane AG, Zurich, Switzerland). The term biovolume refers to the average volume of cells (μm³) in the field observed (14.2 × 10³ µm²) and provides an estimate of the biomass of the biofilm. The percentage of surface covered (%) reflects the effectiveness at colonizing the substrate on the part of the bacterial population. The maximum thickness of the biofilm (μm) was determined directly as a function of the number of optical sections of 1 μm recorded on the z axis. Roughness provided a measure of how much the thickness of the biofilm varied and was, thus, an indicator of biofilm heterogeneity [41]. A roughness with a value of zero indicates a biofilm of uniform thickness, while a value close to one describes a patchy biofilm.

3.3. Statistical Analysis

The quantitative structural parameters of the biofilms were compared by means of analysis of variance (ANOVA) techniques, with separation of averages achieved by the Tuckey test, utilizing the Statistica® 8.0 software package (StatSoft Inc., Tulsa, OK, USA). Significant differences were established for a probability level of 5% (P < 0.05).

4. Conclusions

The ten serotypes of Salmonella tested produced biofilms on polystyrene surfaces, although considerable differences were observed between the various serotypes. The serotypes S. Anatum, S. Hadar, S. Kentucky, and S. Typhimurium showed a trend to have the highest figures for biovolume, percentage of surface covered, and maximum thickness, while the serotypes S. Enteritidis and S. 1,4,(5),12:i:- had the lowest values for these structural parameters. Sodium hypochlorite (with 10% active chlorine; SHY) applied at 50,000 or 100,000 ppm and benzalkonium chloride (BZK) at 5000 and 10,000 ppm were effective in reducing biofilms of most strains of Salmonella (decreasing both their biovolume and their surface coverage). Nonetheless, both biovolumes and percentages of surface coverage increased when the biofilms were exposed to 25,000 ppm of SHY or to 1000 ppm of BZK. This situation was especially marked in the case of BZK, which is a worrying fact in the context of food safety and public health. The work being reported here has contributed to the determination of what amounts of disinfectant are appropriate for eliminating the biofilms of different serotypes of Salmonella enterica. However, results in this research work should be considered with caution because they derive from laboratory-based experiments. Further studies under more realistic conditions should be performed to confirm these findings. Thus, this research study is not aimed at supporting the use of SHY and BZK in the food chain at concentrations other than those included in the recommended use instructions.

Author Contributions

Conceptualization, R.C. and C.A.-C.; Data Curation, R.C. and C.A.-C.; Formal Analysis, C.A.-C.; Funding Acquisition, R.C. and C.A.-C.; Investigation, R.C., S.F.-P., L.B.-D. and C.A.-C.; Methodology, R.C., S.F.-P. and L.B.-D.; Project Administration, R.C. and C.A.-C.; Resources, R.C. and C.A.-C.; Software, R.C.; Supervision, C.A.-C.; Validation, R.C. and C.A.C.; Visualization, R.C. and C.A.-C.; Writing—Original Draft, R.C. and S.F.-P.; Writing—Review & Editing, C.A.-C.

Conflicts of Interest

The authors declare no conflicts of interest.