A Report of Two Cases of Bacteremia by Leuconostoc lactis in Immunocompromised Patients

Background Leuconostoc lactis (L lactis) are gram positive cocci and generally considered to be environmental organisms. They were widely regarded as non-pathogenic and therefore of limited clinical signi�cance. Molecular identi�cation is one technique to identify bacteria. Case reports We presented two cases of bacteremia caused by L lactis. They were both advanced age, history of various diseases, intravenous catheter implantation, parenteral nutrition, longer hospital stay and using of multiple antibiotics. The 16S rRNA genes of the isolates of these two patients´ blood were sequenced. Conclusions When immunocompromised patients with risk factors develop bacteremia caused by L lactis, adequate attention and effective treatment measures should be given.


Background
Leuconostoc lactis (L lactis) are gram positive cocci, adynamic, facultative anaerobic, catalase-negative, belonging to the Leuconostoc spp.It is generally believed that L lactis were not pathogenic to human.In 1984, a study con rmed that leuconostoc was a conditional pathogen which could cause human infections at the rst time (1).Since then, they were considered potentially pathogen.There were various cases of infections caused by the microorganism such as brain abscess (2), urinary tract infections (3), catheter-associated bloodstream infections (4) and sepsis (5) .L lactis were not easily identi ed due to their coccoid morphology in clinical microbiology laboratory.They used to be reported incorrectly as members of the Streptococcus.Due to all clinical isolates of leuconostoc were resistant to vancomycin, it was important to correctly identify the isolated strains and chose appropriate antibiotics.
In this report, we presented two cases of bacteremia by L lactis in immunocompromised patients.The two cases of L lactis were con rmed by molecular identi cation.

Case Reports
Case 1 A-79-yr-old man was admitted to hospital due to weakness of both upper limbs.He had history of alzheimer disease, hypertension and type 2 diabetes for many years.One day after admission, the patient ´s body temperature rose to 38.5℃, white bloodcell (WBC) count, 11.8×10 9 /L (3.5-9.5×10 9 /L, 77.97% segmented neutrophils); C-reactive protein (CRP) level was 9.06mg/L (0-4mg/L) and chest CT showed brosis of the upper right lung with pleural adhesions.These suggested that he might have lung infection.The patient had many diseases with a long medical history.His immunity was low, so the treatment measures were mainly based on symptomatic treatment, while empiric antimicrobial treatment was used.Continuous of empiric antimicrobial treatment with ceftriaxone, meropenem and moxi oxacin.Pseudomonas aeruginosa was isolated from the sputum cultures, antibiotics were changed to tigecycline, imipenem and cilastatin combined with uconazole.However, the patient´s fever persisted and the infection had not resolved.He was transferred to intensive care unit (ICU) due to respiratory failure.Deep venous catheters and tracheotomy were used to meet treatment needs.But chest CT showed his lung infection got worse and CRP level was 43.76mg/L (0-4mg/L).Two samples of blood cultures were positive for bacterial growth, and L lactis was identi ed using the VITEK 2 (bioMérieux, Marcy I´Etoile, France) system.Imipenem and cilastatin combined with linezolid were used according to blood culture results.After a period of treatment, the patient´s infection symptoms were controlled.
Case 2 A-88-yr-old man, with a previous diagnosis of hypertension and lung cancer.This time he was admitted to hospital in ICU for coma after trauma.Brain CT showed bleeding skulls which was the direct cause of his coma.Taking into account the patient´s condition, hemostasis and other symptomatic treatment measures were used after admission.Pseudomonas aeruginosa was isolated in the sputum cultures.According to the results of drug sensitivity test, cefepime, imipenem and cilastatin combined with teicoplanin were used to treat infection until cultures from sputum had no isolates.Since the patient had been in a coma, multiple organ failures started after one month in hospital, tracheotomy and deep venous catheters were used to meet treatment needs.The day after the surgery of venous catheterization, the patient presented a serious infection.His body temperature rose to 39℃, WBC count, 19.2×10 9 /L (89.06% segmented neutrophils); CRP level was 63mg/L (0-4mg/L); procalcitonin level was 1.28ng/ml (<0.5ng/ml).Blood cultures yielded Leuconostoc spp.The pathogen was gram-positive cocci and was identi ed as L lactis.Due to L lactis resistant to vancomycin, piperacillin was used instead of teicoplanin.Then, three blood cultures were taken at different times and all yielded L lactis.Bacteremia caused by L lactis existed persistently.The patient died nally three months after con rmation of L lactis infection.
Microbiological identi cation additional sets of blood cultures were collected to obtain microbiological diagnosis of the two patients, repectively.Bacterium growing in all cultures were catalase-negative, alpha-hemolytic grampositive cocci.The colonies were white, small and raised on blood agar (Figure 1).The bacteria was identi ed as Leuconostoc spp.by VITEK 2 system.The VITEK MS system (bioMérieux, Marcy I´Etoile, France) was used to identi ed this clinical bacterial isolate as L lactis quickly and reliably.To obtain more reliable phenotypic data and species identi cation, the 16S rRNA genes of the isolates of these two patient´s blood were sequenced.All sequences were analyzed by BLAST (basic local alignment search tool) and ribosomal database project.The sequence showed 99% similarity to that of L lactis.Antimicrobial susceptibility tests were detected by diffusion disk method.The microorganism was susceptible to levo oxacin, penicillin, piperacillin-tazobactam, cefoperazone-sulbactam, clindamycin, erythromycin, cefepime and imipenem, but resistant to compound trimethoprim, cefoxitin vancomycin and teicoplanin.

Discussion And Conclusions
Leuconostoc spp.are generally considered to be environmental organisms, which are commonly found on food products and vegetables.They were widely regarded as non-pathogenic and therefore of limited clinical signi cance (6).With the increase of cases of infections caused by Leuconostoc spp, they are now considered to be opportunistic pathogens as a rare cause of bacteremia in severely immunocompromised hosts (7)(8).By studying, we found that decreased immunity, parenteral nutrition, use of certain antibiotics, such as vancomycin and intravenous catheter implantation were considered risk factors for Leuconostoc spp.infection (9-10).
In our report, we presented two cases of bacteremia infection caused by L lactis.Conservative therapy was implemented after they were admitted to hospital.The two patients were extremely ill and did not respond to therapy.The patients' condition had not improved.They had to be ventilated and given central venous catheters to maintain vital signs.There were many similarities between these two patients.They were both advanced age, history of various diseases, intravenous catheter implantation, parenteral nutrition, longer hospital stay and using of multiple antibiotics.Both of them had risk factors for Leuconostoc spp.infection.Bacterial translocation from the skin exposed to venous catheters could be the source of this pathogen.Bacteremia caused by L lactis had not been previously reported in our hospital.
Since Leuconostoc spp.are naturally resistant to vancomycin, we need to consider the bacteremia caused by them and change the antibiotics in time during the treatment of patients with low immunity and intravenous catheter implantation.The correct identi cation of isolates in laboratory could guide clinicians to choose the appropriate antibiotics.The automated identi cation system routinely used in many microbiology laboratories is not reliable to distinguish species.We used the VITEK 2 system to identify bacterial species in our laboratory.In case 2, we misidenti ed the isolate as L mesenteroides through this system.Then, we con rmed the bacterium were L lactis by sequence analysis of 16S rRNA.This method can reliably identify rare bacterial pathogens (11).
It had been reported that the VITEK 2 system could not accurately identify Leuconostoc spp. at the species level.Compared with the partial sequencing of the 16S rRNA gene, the accuracy rate of the VITEK 2 system was only 15%.Through analysis of the 16S rRNA gene sequence, it was found that most of the isolates of L mesenteroides and L pseudomesenteroides identi ed by the VITEK 2 system were L lactis (12).Therefore, we recommended that laboratory personnel should simultaneously perform molecular methods and automated tests to identify Leuconostoc spp more accurately.
We should be consider the bacteremia caused by L lactis among patients with low immunity and intravenous catheter implantation.Meanwhile, it´s important to identify them timely and accurately.The AbbreviationsL Leuconostoc lactis

Figures
Figures

Figure 1 a
Figure 1