Plasmid-encoded multidrug resistance: A case study of Salmonella and Shigella from enteric diarrhea sources among humans

Salmonellosis and shigellosis are significant and persistent causes of diarrheal diseases among humans in developing countries. With that in mind, the current study investigates the occurrence of plasmid-encoded multidrug resistances in Salmonella and Shigella from diarrheal cases among humans. The isolates were characterized by serotyping, antimicrobial-susceptibility testing, transfer experiments and curing. The extended spectrum β-lactamase (ESBL) was detected by the double disc diffusion synergy test (DDST). A significant number of the plasmid-encoded multidrug resistant (PEMDR) Salmonella and Shigella isolates were found to harbour transferable plasmid genes resistant to antibiotics like ampicillin, chloramphenicol, trimethoprim-sulfamethoxazole, ceftriaxone, cefuroxime and to a lesser extent to ciprofloxacin and ofloxacin. The conjugative R-plasmids-encoded extended-spectrum β-lactamase also showed resistances to cephalosporins (ceftriaxone and cefuroxime) and ampicillin. Curing experiments showed chromosomal resistances to varied antibiotics. The findings confirmed the presence of PEMDR in Salmonella and Shigella strains as a suitable adaptation to a changing antibiotic environment. The results therefore suggest the limited use of the commonly prescribed/or third generation cephalosporins as an empirical treatment of multidrug resistant Salmonella and Shigella because this may affect therapeutic outcomes.