A Virulent Strain of Salmonella enterica Serovar London Isolated in Infants with Enteritis Traced by Active Surveillance and Molecular Epidemiological Study

Abstract

A total of 74 isolates of Salmonella enterica serovar London were collected through the Laboratory-Based Diarrheal Diseases Surveillance in [2000][2001]. In order to characterize the isolates and investigate the source of the epidemic, we performed antimicrobial susceptibility tests and XbaI Pulsed-field gel electrophoresis (PFGE) of 44 Salmonella London isolates. Forty isolates were from feces of infants and four isolates were from adults aged 30, 52, 54, and 59 yr. Two subtypes were identified: a tetracycline-susceptible A 0 PFGE pattern and a tetracyclineresistant A 1 PFGE pattern. Interestingly, the isolates from all infants and one 30-yr-old adult were A 0 PFGE pattern and tetracycline-susceptible. Furthermore, the A 0 PFGE pattern strain was approximately 2 times more virulent than the A 1 PFGE pattern strain, according to the results of in vitro invasion assay using J774A.1 macrophage-like cells. These results indicate that the active surveillance with molecular epidemiological tools would be valuable for promptly finding new epidemic strains. Our results also suggested that the virulent Salmonella London strain might infect the infants through a common contaminated source.

Nosocomial Outbreak of Pediatric Gastroenteritis Caused by CTX-M-14-Type Extended-Spectrum β-Lactamase-Producing Strains of Salmonella enterica Serovar London

“…We could not determine the origin of the outbreak. The pattern was also identical to that from community-acquired pediatric patients (8,16), suggesting an identical clone had been spreading. However, it is interesting that our isolates were multidrug resistant, while the other strains reported were susceptible to multiple antimicrobial agents, including ampicillin.…”

“…All three Salmonella isolates from the patients were identified as Salmonella serovar London. However, contrary to the previously reported strains (8), the isolates were resistant to ampicillin, cefotaxime, aminoglycosides, and trimethoprim-sulfamethoxazole. ESBL production was suspected by the double-disk synergy test, and the MICs by Etest, i.e., Ն256 g/ml for cefotaxime, 0.25 g/ml for cefotaxime-clavulanic acid, 3 to 6 g/ml for ceftazidime, and 0.38 g/ml for ceftazidime-clavulanic acid, suggested the enzyme being of the CTX-M type.…”

“…Only 4 of 1,306 (0.3%) Salmonella isolates were of this serovar in 1998 in Korea (7). However, the number increased to 74 in the period from 2000 to 2001 (8), indicating a wide dissemination. In 2000, an infant formula-associated community-acquired gastroenteritis outbreak affecting 31 infants was caused by Salmonella serovar London (16), and even a rare endophthalmitis infection in a previously healthy 3-month-old infant was similarly caused (20).…”

“…Serogroup E Salmonella strains with cefotaxime resistance were isolated from watery stool specimens, and this prompted us to determine a possible relationship between these strains and the recently reported Salmonella serovar London strains (8,16) and to determine the mechanism of ␤-lactam resistance.…”

“…Resistance transfer was tested by both broth-and plate-mating methods using an azide-resistant recipient, Escherichia coli strain J53. Pulsed-field gel electrophoresis (PFGE) of XbaI-restricted genomic DNA was performed according to the manufacturer's instructions (Bio-Rad, Hercules, Calif.), and the patterns were visually compared with those of recently reported Salmonella serovar London strains (8,16).…”