Plasmid-Encoded Multidrug Resistance: A Case study of Salmonella and Shigella from enteric diarrhea sources among humans

Salmonellosis and shigellosis are signifcant 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 signifcant 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 ciprofoxacin and ofoxacin. 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 fndings confrmed 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.

Plasmid-encoded AmpC beta-lactamases: how far have we gone 10 years after the discovery?

The dogma that ampC genes are located exclusively on the chromosome was dominant until about 10 years ago. Since 1989 over 15 different plasmid-encoded AmpC beta-lactamases have been reported from several countries. Most of these enzymes evolved in two clusters. The major cluster includes several enzymes with a high similarity to CMY-2, which is the closest related chromosomal AmpC enzyme of Citrobacter freundii. A second cluster centers around CMY-1. It is less homogeneous and not closely related chromosomal AmpC enzymes. Molecular diversification by amino acid substitutions does not usually translate into a change in the resistance phenotype. At this time, CMY-2 appears to be the most prevalent and widely distributed. Further global increase of prevalence and diversity of plasmidic AmpC beta-lactamases have to be anticipated in the next millenium.