Characterization of Leuconostoc lactis Bacteremia during a 2-year Study at a Tertiary Care Center in North India—An Observational Analysis

ABSTRACT.

Leuconostoc species are regarded as important causes for many infections in immunocompromised patients. In this study, we assessed the characteristics of Leuconostoc spp. causing bacteremia in patients at our center. This observational analysis was conducted in the microbiology laboratory of a tertiary care center in northern India from July 2021 to July 2023. Patients in whom blood culture bottles were positive for Leuconostoc lactis were included in the study. Culture isolates were identified by MALDI-ToF MS as L. lactis and tested for antibiotic sensitivity results by Kirby–Bauer disk diffusion method. Demographic and clinical details were collected and analyzed. During the study period, 6,742 blood culture bottles flagged positive. Among these, L. lactis was isolated from 14 (0.21%) patients. The median patient age was 34 years. The male-to-female ratio was 2.5:1. All the patients with L. lactis bacteremia had an underlying condition leading to immunosuppression (e.g., carcinoma and chronic kidney disease). All the patients with L. lactis bacteremia had an intravascular device present at the time of bacteremia. All isolates in the study were sensitive to doxycycline, high level gentamicin, minocycline, ampicillin-sulbactam, and linezolid. Mortality was attributed to bacteremia by L. lactis in five patients. Appropriate and timely identification of the Leuconostoc species is important for the clinician to tailor regimens for the patients.

INTRODUCTION

Leuconostoc species are facultatively anaerobic Gram-positive cocci, catalase-negative organisms.^1–3 They were previously classified under the family Streptococcaceae but now belong to the family Leuconostocaceae and the order Lactobacillales.⁴ They are commonly used in the food industry as probiotics.^1,3 The first case of Leuconostoc documented in the literature was initially mistakenly attributed to Streptococcus sanguinis in 1985 in France.⁵ Previously, Leuconostoc species were considered nonpathogenic to humans; however, recent research has proven the organism to produce ventriculitis,⁶ osteomyelitis,⁷ meningitis,^1,2 pneumonia,³ abdominal abscess,⁸ and bacteremia.^9,10 Other associated risk factors have been observed in individuals with malignancy.¹¹ Chronic liver failure⁸ and chronic renal illness^1,12 have also been identified as risk factors. The species is predominantly associated with device-related infections in immunocompromised individuals. Long-term usage of antibiotics, such as beta-lactam and vancomycin,⁵ pose a risk of infection with these bacteria. Although they have largely been discovered in patients with weakened immune systems, the fatality of infection is seldom observed. Generally, the preferred antibiotic prophylaxis against the Gram-positive organisms is vancomycin. However, because intrinsic resistance to vancomycin is a salient feature of Leuconostoc spp.,² it becomes essential to identify the organism correctly and prevent morbidity with appropriate therapy. It is vital to understand the clinic-microbiological profile of the organism to avoid misuse of antibiotics and curb the rising drug resistance strains. Herein, we highlight the characteristics of Leuconostoc spp. causing bacteremia in patients seeking healthcare at our center. The study addresses the patients’ clinical profiles, laboratory parameters, and risk factors associated with Leuconostoc spp. bacteremia.

MATERIALS AND METHODS

This observational analysis was conducted in the microbiology laboratory of our institution from July 2021 to July 2023. Patients in whom blood culture bottles were positive for L. lactis were included in the study. Two sets of blood cultures were sent to the microbiology laboratory for culture and sensitivity. If both sets showed growth of L. lactis, then they were included in the study. The blood culture bottles were incubated in a BacTAlert machine as soon as they reached laboratory. Blood samples from positive bottles were cultured on 5% Blood agar and MacConkey agar and incubated for 24 hours. A Gram stain was also prepared, which showed Gram positive cocci in pairs and short chains (Figure 1). After incubation the colonies on the plate (Figure 2) were identified by MALDI-ToF MS. The isolates identified as L. lactis were further tested for antibiotic sensitivity. Inoculum was prepared from the 24-hour growth on a blood agar plate by suspending three to four morphologically similar colonies in tryptic soy broth. Inoculum was adjusted to 0.5 McFarland standards. Antimicrobial sensitivity testing was performed by Kirby–Bauer disk diffusion method. The susceptibility results were read and interpreted after 18 to 24 hours of incubation using Clinical and Laboratory Standards Institute 2023 M-45 clinical breakpoints. The antibiotics tested were ampicillin (10 µg), ampicillin–sulbactam (10/10 µg), high-level gentamicin (120 µg), doxycycline (30 µg), minocycline (30 µg), levofloxacin (5 µg), vancomycin (30 µg), linezolid (30 µg), chloramphenicol (30 µg), and teicoplanin (30 µg). The zone diameters were interpreted according to the literature by Yang et al.³ Demographic and clinical details were collected and analyzed. Variables were calculated as median values.

Figure 1.

Gram-stained smear of the blood cultures positive for Leuconostoc lactis showing gram positive cocci in pairs and short chains.

Figure 2.

Colonies of Leuconostoc lactis on 5% blood agar. The culture plate showed growth of small opaque nonhemolytic colonies of 1- to 2-mm diameter, after 24 hours of incubation.

RESULTS AND DISCUSSION

During the study period, 53,672 blood culture bottles were received in the bacteriology laboratory of our department. Of these, 6,742 (12.56%) bottles flagged positive. Among these, L. lactis was isolated from 14 (0.21%) patients. This high incidence of L. lactis cases in our institution might be because our center is a referral center. Most of the cases admitted here are referred cases in serious condition. Most the patients have already been administered with empirical antibiotics before admission to our institution. This might be the reason for resistance among the Leuconostoc species. It was discovered that the administration of broad-spectrum antibiotics rendered patients more likely to isolate previously uncommon pathogens. An increase in the isolation of vancomycin-resistant bacteria, especially Leuconostoc species, may be caused by the increased use of vancomycin.⁵

The median age of the patients was 34 years (range: 26–74 years). The male-to-female ratio was 2.5:1. All the patients had high-grade fever at the time of sample collection. The major risk factors and underlying comorbidities are shown in Table 1. All the patients with L. lactis bacteremia had an underlying condition leading to immunosuppression such as carcinoma, chronic kidney disease, and pancreatitis (Table 1). These conditions demand the administration of chemotherapy and steroids, which increases the risk of bacteremia. In a review by Cuervo et al.,¹¹ he described a series of cancer patients with Leuconostoc bacteremia. From Table 1, it can be seen that all the patients with L. lactis bacteremia had an intravascular device present at the time of bacteremia. Fever along with the presence of an intravascular device is a characteristic presentation in L. lactis bacteremia patients.⁹ A case report by Modaweb et al.⁵ described the case of a child with biliary atresia who had infection of central venous catheter caused by Leuconostoc species. Removal of the catheter helped in speedy recovery of the child. Also, three patients had mixed infection in blood along with L. lactis. Klebsiella pneumoniae, Ralstonia mannitolilytica, and Chryseobacterium gleum were isolated in three patients respectively, along with L. lactis. Leuconostoc species are thought to have a minimal potential for disease in healthy people, but they have recently been recognized as pathogens in immunosuppressed hosts,¹³ in whom they can cause fatal diseases such as sepsis or meningitis.¹⁰ Increased intestinal permeability and intestinal immunological abnormalities are known to produce bacterial translocation in liver cirrhosis patients, and this mechanism is one of the main reasons for systemic infections.¹

Table 1.

Risk factors and underlying comorbidities in patients with Leuconostoc lactis bacteremia

Risk Factors and Comorbidities	Number of Patients with L. lactis (N = 14)
Underlying comorbidities
Carcinoma	4
Pancreatitis	5
Septic shock	3
Chronic kidney disease	1
Cerebrovascular accident	1
Mixed infection in blood	3
Risk factors
Length of hospital stay >30 days	8
On chemotherapy	4
On steroids	6
Diabetes mellitus	3
Hypertension	2
Hypothyroidism	2
Procalcitonin value of >5 ng/mL	14
Presence of intravascular device	14

A study by Lee et al. is considered the largest case series of Leuconostoc species infection and describes 20 patients. Of these, 19 patients had healthcare-associated infection, 11 patients had an underlying malignancy, 11 patients were hospitalized for more than 30 days before the onset of bacteremia. Many other risk factors have also been described in the study by Lee et al.¹⁴ These include recent chemotherapy (n = 9) and prior corticosteroids (n = 7).¹⁴ The risk factors described in a study by Handwerger et al.¹⁵ include previous antibiotic use, procedures that disrupted integumentary defenses, and polymicrobial infection. A study was conducted by Florescu et al.,¹⁶ on six pediatric patients with short bowel syndrome who were diagnosed with Leuconostoc bacteremia. The authors documented the risk factors such as total parenteral nutrition, presence of central lines, and disrupted bowel mucosa.

Table 2 shows the sensitivity pattern of the isolates of L. lactis included in the study. The isolate is intrinsically resistant to vancomycin and teicoplanin. All isolates in the study were sensitive to doxycycline, high-level gentamicin, minocycline, ampicillin-sulbactam, and linezolid. The choice of antibiotic is important when treating infections brought on by L. lactis. This organism typically responds to penicillin, but unlike other Gram-positive organisms, it has a characteristic vancomycin resistance due to the production of peptidoglycan precursors with the D-Ala-D-Ala amino acid sequence.¹ Because the use of vancomycin is restricted in this organism, the appropriate alternative option can only be decided by antibiotic sensitivity testing. The most susceptible antibiotics in this study were penicillin groups of antibiotics and tetracycline group of antibiotics. Mortality was attributed to bacteremia by L. lactis in five patients (35.71%). This high incidence of mortality could also be due to the high rate of referred cases with severe immunocompromised conditions.

Table 2.

Sensitivity pattern of the Leuconostoc lactis isolates

Antibiotics	No. of Isolates of L. lactis with Sensitive Pattern
Ampicillin	13
Doxycycline	14
Erythromycin	2
High-level gentamicin	14
Levofloxacin	10
Linezolid	14
Minocycline	14
Ampicillin–sulbactam	14
Chloramphenicol	1
Vancomycin	0
Teicoplanin	0

CONCLUSION

High prevalence of L. lactis infection at our institute emphasizes the fact that empirical treatment with vancomycin might not be effective in all Gram-positive organisms. Routine treatment protocol of Gram-positive organisms is not effective in the case of Leuconostoc species because the latter are intrinsically resistant to vancomycin. Therefore, appropriate and timely identification of the Leuconostoc species is important for the clinician to tailor regimens for the patients.