Abstract:
Globally, non-typhoidal salmonellosis accounts for approximately 80.3 million cases of
human infections annually. Estimates of salmonellosis due to consumption of pork or
pork products is difficult to determine but, it ranges from < 1% to 25%. These invasive
pathogens colonize intestinal mucosal surface but, they are self-limiting in health
individuals due to a noble immunity. Utilization of antimicrobial agents in pig farming
has been associated with the spread of resistant Salmonella species to man and the
carrier status presents a major hazard to human health. This study examined 54
isolates for antimicrobial resistance, sequenced seven housekeeping genes, and
performed Multi-Locus Sequence Type (MLST) analysis. We detected β-lactamase and
tetA(B) genes in 100% and 80% of the isolates respectively. Data analysis using Codonbased
Test of Neutrality analysis between sequences revealed P-value less than 0.05,
an indication of strong forces of natural selection pressure acting at the sequence type
level. Further data analysis using the Maximum Composite Likelihood Estimate of the
Pattern of Nucleotide Substitution discovered frequencies of 0.177 (A), 0.244 (T/U),
0.263 (C), and 0.317 (G). The transition/transversion rate ratios were found to be k1 =
2.698 (purines) and k2 = 20.089 (pyrimidines) with an overall transition/transversion
bias of R = 6.565, where R = [A*G*k1 + T*C*k2]/[(A+G)*(T+C)] further confirming that
indeed the Salmonella isolates studied here were divergent. These results suggest that,
Salmonella isolates of sequence types (STs) coexist in the intestine thereby providing
for an efficient intestinal colonization and multiple adaptations. Our results offer
general and rapid approaches for identifying genetic diversity of Salmonella serotypes
in individual pig carcasses which can be adopted for molecular epidemiological surveys
of important food contaminating bacterial pathogens.
