Bacillus Spp. isolated from the conjunctiva and their potential antimicrobial activity against other eye pathogens

Abstract

Background

In this study, we attempted to screen and investigate antibacterial activity of Bacillus species, which were isolated from conjunctiva, against other eyes pathogens.

Methods

To examine predominant isolates of Bacillus subtilis, B. pumilus, B. cereus and B. mojevensis, isolated from conjunctiva for their antimicrobial activity against indicator microorganisms as Micrococcus luteus, Staphyloccocus aureus, S. epidermidis, S.hominis, S. lugdunensis, S.warneri, S. haemolyticus, B. cereus, Listeria monocytogenes, and Proteus mirabilis. Growth inhibitions of indicator microorganisms were tested using agar diffusion tests by cells and supernatants of five B. mojevensis, one B. subtilis, four B. cereus and five B. pumilus strains which were isolated from conjunctiva.

Results

The Bacillus isolates showed variable ability of inhibition against the tested microorganisms. Two strains of B. pumillus, 1 strain of B. subtilis, 5 strains of B. mojevensis, 1 strain of B. cereus were efficacious against the tested microorganisms. Most resistant microorganism to these bacteria was Proteus mirabilis. Two of Gram positive bacteria, S. lugdenensis (K15-9) and S. aureus (SDA48), were also found as resistant.

Conclusions

In this study, Bacillus spp isolated from conjunctiva showed antimicrobial activity against Gram-positive bacteria. Human eye-derived microorganisms and their antimicrobial effects might be a useful source of natural products for the future.

Introduction

Application of antibiotics in the treatment of bacterial disease hasbeen a noticeable medical success in this century. However, gradual emergence and spread of antibiotic resistance among bacterial population due to wrong or excessive use of antibiotics has led to the development of public health problems.

Bacillus genus is made up of Gram positive aerobic or facultative endospore forming rod shaped bacteria. Bacteria of the genus Bacillus are known to produce natural products (1). They possess antagonistic activities against many bacterial and fungal pathogens and are often used as agents for the treatment and/or prevention of different plant and animal infections.

Their antimicrobial activities have mainly been attributed to the production of antibiotic peptide derivatives as bacteriocins and bacteriocin-like inhibitory substance, lipopeptides, which have powerful surfactant like properties with numerous biotechnological applications including deemulsification, health care, and food industry (1) Bacillus spp. can produce antibiotics which are in peptid structure, such as bacitracin, polymyxin, tyrosidin, grmysidin, subtilin and sirkulin. For this reason, they have an important role in drug industry. Their antimicrobial activities have mainly been attributed to the production of antibiotic peptide derivatives and lipopeptides (1–3).

Coagulase- negative Staphylococcus (CNS) causes the vast majority of post- operative endophtalmitis cases. Intraocular infections with S. aureus, enterococci, Bacillus or Gram negative species are often intractable. Because of these, blindness or loss of the eye itself is not uncommon (4,5). Resistance to antibiotics in CNS is major concern. Penicillin resistance in CNS is very high (6,7). Methicillin-resistant Staphylococcus species, especially S. aureus strains, appeared in the hospital environment and acquired resistance not only to β-lactam antibiotics but also to flouroquinolones, chloramphenicol, clindamycin, tetracycline, and aminoglycosides (8). Recently, a decrease in the susceptibility of methicillin-resistant Staphylococcus species to vancomycin and teicoplanin has also been reported in several hospitals around the world (9–10).

The need for antibiotics and antimicrobials continues to be a major challenge for the treatment of infectious disease which affect millions of people worldwide. Furthermore, antimicrobial resistance is a growing concern. Also, the number of resistant bacteria and the geographic distribution of these organisms are both rising. We will need new products against these organisms. Human eye-derived microorganisms might be a useful source of natural products.

In this study, we screened Bacillus spp. strains, which were isolated from human eyes, for their antibacterial activity against human eye pathogens.

Material and methods

Bacterial cultures

Several Bacillus spp. were used as the producer strain. Strains of Bacillus spp. which were isolated previously from healthy eyes and stored as pure state were chosen. The nutrient agar was used for maintenance of the strain with 20% (v/v) glycerol at −86°C.

In this study, eye pathogens (Staphylococcus aureus SDA 40.2, SDA 48, Staphylococcus epidermidis KA 11.1, KA 14.1, KA 17.1, SDA 44, Staphylococcus warneri PCA 9.5, KA 11.2, Staphylococcus hominis PCA 6.3, PCA 9.2, PCA 9.3, Staphylococcus lugdunensis PCA 7.2, KA 15.9, Micrococcus luteus PCA 7.1, Enterococcus faecalis PCA 39.1.1, Bacillus cereus 13.2 PCA, Listeria monocytogenes 47 PCA and Proteus mirabilis KA 44.1 ) were used as test bacteria.

Screening for antimicrobial activity by cross-streak method

In primary screening, all Bacillus spp. isolates were streaked as a straight line at the centre of agar plates. These plates were incubated at 37°C for 24 hours. On the incubation, tested bacteria were inoculated using a single streak that was perpendicular to the Bacillus growth streaked at single straight line at the centre of the plate followed by incubation at 37°C for 24 hours. Inhibition zones formed were measured in millimeter (11).

Antimicrobial activity of cell free supernatant by well diffusion method

To extract bioactive compound from culture supernatant during the growth cycle, 28 producer strains Bacillus were separately inoculated using 200 ml sterile nutrient broth (NB) and incubated on a shaker at 37°C overnight 120 rpm for 48 hours. Cells were collected from a 48 hours culture by centrifugation (6000 rpm for 20 min, at 4°C) and the supernatant recovered and passed through a 0.22 µm filter.

The determination of the inhibitory effect of cell free supernatant of isolates on test bacteria was carried out according to the well diffusion method. Pre-poured agar media plates equilibrated were spread with 10⁶ cfu/ml of respective test organism and allowed to dry. In the agar plates, wells of 6 mm diameter were cut using a cork borer. The wells were filled with 80 µl of cell free culture supernatant and incubated overnight at 37°C. The plates were then examined for clear zones of inhibition surrounding each well and inhibition zones were measured.

The test was duplicated for each Bacillus isolate.

Partial purification bioactive compound

The isolates were inoculated into flasks containing 100 ml nutrient broth and incubated at 37°C in a shaker at 120 rpm for 48 hours. After growth, culture media were centrifuged at 6000 rpm for 20 min, at 4°C and the supernatant recovered and passed through a 0.22 µm filter. Cell-free culture was extracted 3 times with an equal volume of ethyl acetate according to the method of Han et al. (12). Ethyl acetate was added to the supernatant in ratio of 1:1(v/v). The mixture was shaken vigorously for 10 min and separated. Subsequently, the ethyl acetate extract was pooled and evaporated to dryness under vacuum at 60°C for 20 min in water bath (13).

After extraction, bioactive compound, which were antibacterial activity, were chosen using disc diffusion methods. A 20-ml aliquot of partial purification bioactive compound was applied to disks (6 mm) placed on agar plates previously inoculated with a suspension of each eye pathogens. The plates were incubated at the 37°C for 24 hours. The diameter of inhibition zones was measured.

MIC test was applied to the partial purification bioactive compound, which had most antimicrobial activity. Therefore, Minimum inhibitory (MIC) values were determined using the method of two fold serial dilution (14).

Antibiotic Susceptibility Testing

Antimicrobial resistance patterns of test bacteria were determined by the agar disk diffusion method. Disks containing the following antibacterial agents were used: gatifloxacin (5 µg), cefuroxime (30 µg), ceftazdime (30 µg), vancomycin (30 µg), gentamicin (10 µg), amikacin (30 µg), ciprofloxacin (5 µg), lomefloxacin (10 µg), moxifloxacin (5 µg), methicillin (10 U). Characterization of strains as sensitive, intermediate or resistant was based on the size of the inhibition zones around each disk according to the National Committee for Clinical Laboratory Standards (CLSI) criteria (15).

Results

Screening for antimicrobial activity using cross-streak method

Twenty eight strains of Bacillus sp. which were isolated previously from healthy eyes were included in the study. These isolates were screened for antimicrobial activity against eye pathogens using cross-streak method. Therefore, it was noticed that 11 of 28 strains had antimicrobial activity against one or more eye pathogens.

Antimicrobial activity of cell free supernatant by well diffusion method

Cell free supernatant (CFS) of culture of 11 isolates were screened for antimicrobial activity against eye pathogens. The antimicrobial spectrum of cell free supernatant was given in Table 1.

Table 1.

Antibacterial activity of cell free supernatant against tested bacteria by the well diffusion test (mm)

Test bacteria	B. pumilusPCA 4.2	B. subtilisPCA 11.2	B. cereusPCA 15.3	B. mojavensis3 PCA	B. mojavensis3M17	B. mojavensisKA 39.3
S. hominisPCA 9.2	12	10	12	14	14	12
S. hominisPCA 9.3
S. warneriPCA 9.5	10	12	10	10	12	12
B. cereus13.2 PCA				16
S. epidermidisKA 11.1	12			8
S. epidermidisKA 14. 1				14		10
S. lugdunensis KA 15.9				12
P. mirabilisKA 44.1	12	10	10	10	10	10
S. aureusSDA 40.2	12	12	12	14	14	12
S. epidermidisSDA 44	14	12	14	14	14	14

From this screening, 7 Bacillus sp. showed antimicrobial activity against test bacteria. S. aureus SDA 48, S. epidermidis KA 17.1, S. warneri KA 11.2, S. hominis PCA 6.3, S. lugdunensis PCA 7.2, M. luteus PCA 7.1, E. faecalis PCA 39.1.1, B. cereus 13.2 PCA, L.monocytogenes 47 PCA were found as resistant to all the cell free supernatant. While, S. hominis PCA 9.2, S. warneri PCA 9.5, P. mirabilis KA 44.1, S. aureus SDA 40.2 and S. epidermidis SDA 44 were found the most sensitive to the cell free supernatant.

Antimicrobial activity of partial purification bioactive compound

Partial purification of bioactive compound from liquid culture of Bacillus spp. was carried out. The antimicrobial spectrum of partial purification supernatant is given in Table 2. Nevertheless, Inhibitory activity observed on S. hominis PCA 9.2, S. aureus SDA 40.2, S. warneri PCA 9.5, S. warneri KA 11.2, B. cereus 13.2 PCA, L.monocytogenes 47 PCA, S.epidermidis KA 11.1, KA 14.1, KA 17.1, S. lugdunensis KA 15.9 , M.luteus PCA 7.1 and E. faecalis PCA 39. 1.1 were not inhibited.

Table 2.

Antibacterial activity of partial purification bioactive compound against tested eye pathogens by the disk diffusion test (mm).

	Eye Pathogens	S. hominisPCA 6.3	M. luteusPCA 7.1	S. lugdunensisPCA 7.2	S. hominisPCA 9.2	S. hominisPCA 9.3	S. warneriPCA9.5	E. faecalisPCA 39. 1.1	B. cereus13.2 PCA	L.monocytogenes47 PCA	S. epidermidisKA 11.1	S. warneriKA 11.2	S. epidermidisKA 14.1	S. lugdunensisKA15.9	S. epidermidisKA 17.1	P. mirabilisKA 44.1	S. aureusSDA 40.2	S. epidermidisSDA 44	S. aureusSDA 48
B. pumilusPCA 4.2			nt	nt	10					10	10	12	10	10
B. pumilusPCA 9.4			nt	nt	10		12		12	10	10	10	12	10		10
B. subtilis PCA 11.2		10	nt	nt	14		12		12	10	10	12	10	10		12	10	12	12
B. cereusPCA 15.3			nt	nt	10					10				10		10
B. mojavensis PCA 24.1			nt	nt	10		14			10	10	12		10
B. mojavensis 3 PCA			nt	nt	10		14		12	10	14	14	14	10		12		10	10
B. mojavensis 3M17			nt	nt	12	12	14		10	10			10	10		10	10
B. mojavensis KA 39.3			nt	nt	10					10		12		10		10
B. mojavensis 25-2-C2PX			nt	nt	10	12	14			10				10		10

nt, not tested

The MICs were evaluated for all partial purification of bioactive compound MIC. B. mojavensis 3 PCA in Partial purification of bioactive compound showed the lowest MIC value. Although, there is interesting selectivity with good activity against MRSA S. aureus SDA 40.2 and S. epidermidis SDA 44 (4.31µg/ml), yet no activity against S.hominis PCA 9.2, B. cereus 13.2 PCA, L. monocytogenes 47 PCA and S. aureus SDA 40.2. While, these bacteria may need more substance of higher concentration. It was noticed that, MIC values of the other pathogens were found 8.63µg/ml. Notwithstanding, as we could not get enough substance from other strains so we could not find MIC values.

Antibiotic Susceptibility tested eye pathogens

Antibiotic resistance patterns of the eye pathogens were summarized in Table 3. Two S. aureus, five CNS cultures showed methicillin resistance. Three culture CNS showed no drug resistance.

Table 3.

Sensitivities of eye pathogens to commonly used antibiotics (mm).

Test Bacteria	Cefuroxime (30 µg)	Methicillin (10 U)	Ceftazidim (30 µg)	Ciprofloxacin (5 µg)	Gentamicin 10µg)	Amikacin (30 µg)	Vancomycin (30 µg)	Gatifloxacin (5 µg)	Lomefloxacin (10 µg)	Montifloxacin (5 µg)
E. faecalisPCA 39.1.1	R	R	R	21	14	14	17	23	R	11
S. aureus SDA 48	14	R	19	23	20	R	7	R	26	R
S. aureus SDA 40.2	16	R	20	23	21	24	R	32	22	40
S. epidermidis SDA 44	30	16	22	24	25	14	19	21	20	26
S. epidermidis KA 17.1	R	R	R	R	30	R	17	11	R	20
S. epidermidis KA 14.1	30	R	20	R	26	21	23	26	R	23
S. epidermidis KA 11.1	16	R	23	35	14	14	R	32	35	26
S. warneri PCA 9.5	R	R	R	30	19	20	23	30	21	27
S. warneriKA 11.2	35	25	25	27	27	24	22	31	26	21
S. hominis PCA 9.3	24	21	21	30	20	27	15	11	36	21
S. hominis PCA 9.2	42	30	34	30	32	40	21	37	34	40
S. hominis PCA 6.3	46	30	26	37	44	33	11	40	43	36
S. lugdunensis KA 15.9	51	26	21	40	31	30	30	46	56	43
S. lugdunensis 4PCA 7.2	11	R	R	30	22	20	19	37	40	32
P. mirabilisKA 44.1	26	16	19	33	21	14	20	52	R	19
M. luteus PCA 7.1	40	12	26	26	32	21	21	31	22	27
B. cereus13.2 PCA	R	R	R	S	21	26	20	35	40	42
L.monocytogenes47 PCA	R	R	12	30	21	26	21	40	26	26

Discussion

There are many species of the genus Bacillus which can produce a wide variety of antibiotics including bacitracin, polymyxin, colistin etc. On this note, several bacitracins were characterized; the bacitracin A is the commercial product (16).

The present research work was carried out using bioactive metabolites obtained from B. pumilus PCA 4.2, PCA 9.4, B. subtilis PCA 11.2, B. cereusPCA 15.3, B. mojavensis PCA 24.1, 3PCA, 3M17, KA39.3 and 25-2-C2PX.

The antibacterial activity of the Bacillus spp. was tested against different eye pathogens. These eye pathogens were Methicillin resistant S. aureus SDA 40.2 and S. aureus SDA 48, S. epidermidis KA 11.1, S. epidermidis KA 14.1, S. epidermidis KA 17.1, S. warneri PCA 9.5, S. lugdunensis PCA 7.2 while, other eye pathogens were S. epidermidis SDA 44, S. warneri KA 11.2 S. hominis PCA 6.3, S. hominis PCA 9.2, S. hominis PCA 9.3, S. lugdunensis KA 15.9, M. luteus PCA 7.1, E. faecalis PCA 39.1.1, B. cereus 13.2 PCA, L.monocytogenes47 PCA and P. mirabilis KA 44.1.

Antimicrobial activity of Bacillus spp. was noticed from the work of other researchers. Oscariz and Pisabarro (17) isolated and identified cerein 7, which was a bacteriocin produced by B. cereus Bc7 and inhibits growth of Listeria spp. and other gram-positive bacteria. Bizani and Brandelli (18) isolated and identified cerein 8A. That was a bacteriocin produced by B. cereus that inhibits growth of Listeria spp. and M. luteus. Antibiotic production abilities of B. subtilis, B. polymyxa and B. brevis, B. licheniformis, B. cereus were showed by Yılmaz and Beyatlı (19). In our study, antimicrobial activity was noticed in 1 strain of B. subtilis and 1 strain of B.cereus. B.cereus partial purification bioactive compound had been effective over S.hominis PCA 9.2, S. lugdunensis KA 15.9, L.monocytogenes 47 PCA and P. mirabilis KA 44.1. Also, B. subtilis, showed effectiveness over test bacteria except S.hominis PCA 9.3, S. epidermidis KA 17.1 and E. faecalis PCA 39.1.1. Bacillus cereus produces several bacteriocin-like inhibitory substances (1). Tabarez et al. (20) reported an antimicrobial activity of substances produced by B.subtilis(soil isolate) against multidrug-resistant bacterial pathogen including methicillin-resistant S. aureus. El-Bana et al., (21) reported preliminary antimicrobial activity of substances produced by Bacillus subtilis NB-6 (air flora isolate), Bacillus megaterium NB-3 (air flora isolate) against a number of methicillin-resistant Staphylococcus aureus (MRSA). Effective antimicrobial compounds with a broad spectrum of activity against Gram positive and Gram-negative bacteria, and also against methicillin- resistant Staphylococcus clinical isolates (S. hominis, S. epidermidis, S. aureus, S. haemolyticus, S. warneri, S. cohinii and S. scuiri), were secreted by B. subtilis B38 strain into the culture medium (9). In this study, any of the biological active matters, which were tested, were not effected over E. faecalis PCA 39.1.1 and S. epidermidis KA 17.1. Accordingly, these pathogens showed resistance to antibiotics which are commonly used.

In this study, B. pumilus PCA 4.2 and B. pumilus PCA 9.4 showed antibacterial activity against methicillin resistant S. epidermidis KA 11.1 and S. epidermidis 14.1. Moreover, in one of the studies, B.pumilus showed surfactin production (22) and in another study B.subtilis and B.pumilus showed antibacterial activity against many Gram negative and Gram positive bacteria (23). Hence, B. pumilus produces plasmid-encoded peptide pumilicins (1). However, pumilicins show remarkable antibacterial activity against MRSA, vancomycin-resistant E. faecalis (VRE) and several Gram-positive test bacteria (24). This not with standing, Awais et al. (25) studied inhibitory effects of a Bacillus sp. isolate on 2 pathogenic strains of M.luteus and S.aureus. In addition, Hasan et al. (26) have reported a compound produced by B. pumilus that inhibits M.luteus and S. aureus. Ma et al. (27) isolated three lipopeptids from B. mojavensis B0621A. One of these lipopeptids, which was identified as iturinic lipopeptid, showed bioactivity. This was named as mojavensis A. We found antimicrobial activity in 4 of 5 strains of B. mojavensis. Similar results were found by Kim et al., (28). Reportedly, mersacidin produced by Bacillus sp. HIL Y-85, 54728 inhibited the growth and colonization of methicillin resistant S.aureus (29).

Therefore, the identity of the bioactive compound produced by the Bacillus sp. is still unknown. Thus, further analysis by protein electrophoresis and MS/MS mass spectrometry may help to reveal the identity of the protein.

Conclusions

Bacillus spp. has been considered as a potential agent to control against eye pathogens. The bacteriocin or bacteriocin-like inhibitory substance, lipopeptides produced by the strain B. mojavensis 3PCA and B.subtilis PCA 11.2 may represent an antimicrobial substance with potential application in the prevention and treatment of eye infection. Although some bacteriocins from Bacillus present a narrow antimicrobial spectrum, the antibacterial activity of B. mojavensis 3PCA and B. subtilis PCA 11.2 were comparable to broad-range bioactive compound associated with Bacillus spp.

In this study, we have described the antimicrobial activity of substances produced by B.subtilis, B. pumulis and B. mojavensis against several methicillin-resistant S. aureus, S. epidermis and S. warneri strain which may serve as a promising development for new drugs against microbial pathogens. The inhibition of bacterial strains (methicillin-resistant S. aureus S. epidermidis and S. warneri) using B. subtilis PCA 11.2 and B. mojavensis 3 PCA may represent an antimicrobial substance with potential application in the prevention and treatment of eye infection and may be proposed as an alternative strategy for infection control.