Characterization of Invasive Salmonella Serogroup C1 Infections in Mali

Abstract

Nontyphoidal Salmonella (NTS) are the leading cause of foodborne infections worldwide and a major cause of bloodstream infections in infants and HIV-infected adults in sub-Saharan Africa (SSA). Salmonella Typhimurium (serogroup B) and Salmonella Enteritidis (serogroup D) are the most common serovars in this region. However, data describing rarer invasive NTS serovars, particularly those belonging to serogroups C1 and C2, circulating in SSA are lacking. We previously conducted systematic blood culture surveillance on pediatric patients in Bamako, Mali, from 2002 to 2014, and the results showed that serovars Typhimurium and Enteritidis accounted for 32% and 36% of isolates, respectively. Here, we present data on 27 Salmonella serogroup C1 strains that were isolated during this previous study. The strains were typed by serum agglutination and multilocus sequence typing (MLST). Sixteen strains were Salmonella Paratyphi C, four were Salmonella Colindale, and two were Salmonella Virchow. Interestingly, five strains were identified as the very rare Salmonella Brazzaville using a combination of serum agglutination and flagellin gene typing. Phenotypic characterization showed that Salmonella Brazzaville produced biofilm and exhibited catalase activity, which were not statistically different from the gastroenteritis-associated Salmonella Typhimurium sequence type (ST) 19. All tested Salmonella Paratyphi C strains were poor biofilm producers and showed significantly less catalase activity than Salmonella Typhimurium ST19. Overall, our study provides insight into the Salmonella serogroup C1 serovars that cause invasive disease in infants in Mali. In addition, we show that MLST and flagellin gene sequencing, in association with traditional serum agglutination, are invaluable tools to help identify rare Salmonella serovars.

Figure 1.

Number of isolates of each Salmonella serovar by year. Isolates were recovered from the blood of patients and assigned to a serovar by serum agglutination, multilocus sequence typing, and flagellin gene sequencing.

Figure 2.

Distribution of Salmonella serogroup C1 nontyphoidal Salmonella cases by age (A) and serovar (B). *P = 0.03 (two-tailed t test).

Figure 3.

Biofilm formation in microtiter plates. The amount of biofilm formation was evaluated by the crystal violet binding assay and quantified by absorbance at 600 nm. Error bars indicate standard error of the mean, from at least four independent experiments. Asterisks indicate significant difference with S. Typhimurium I77. ***P < 0.001, n.s. = not significant (two-tailed t test).

Figure 4.

Catalase activity of various Salmonella strains. Catalase activity was evaluated for each strain in three independent experiments. Error bars indicate standard error of the mean. Asterisks indicate significant difference with S. Typhimurium I77. ***P < 0.001, n.s. = not significant (two-tailed t test).