Plasmid-mediated Resistance Genes Research Articles

Molecular characterisation and mechanisms of resistance of multidrug-resistant human Salmonella enterica serovar Typhimurium isolated in Amiens (France)

Antimicrobial resistance patterns of Salmonella enterica serovar Typhimurium isolates obtained during the study period were examined. The molecular epidemiology and the mechanisms of resistance to ampicillin, chloramphenicol and tetracycline were investigated. Resistance to ampicillin increased from 59% between 1996 and 1999 to 62.5% in 2000 and to 66.6% in 2001. Of 51 S. Typhimurium isolates studied, 100% were resistant to ampicillin (minimum inhibitory concentration (MIC) >256 mg/L) and sulphonamide (MIC range, 128 to >256 mg/L). Ninety-eight percent of isolates were resistant to streptomycin (MIC range, 48–256 mg/L), 92.2% to tetracycline (MIC range, 32 to >256 mg/L), 88.2% to chloramphenicol (MIC > 256 mg/L), 21.5% to sulphamethoxazole/trimethoprim (MIC > 32 mg/L), 5.8% to amoxicillin/clavulanic acid (MIC, 32 mg/L) and 1.9% to cefalothin (MIC, 64 mg/L). Six resistance phenotypes were found (a–f), with phenotypes a (47%) and b (27.5%) being predominant. Twenty-five (49%) of 51 isolates produced a single β-lactamase, among which 48% produced PSE-1, 44% produced TEM-1 and 8% produced OXA-1. Among 26 of the 51 isolates, 10 produced PSE-1 + OXA-1, 7 produced TEM-1 + PSE-1 + OXA-1, 6 produced TEM-1 + PSE-1, and 3 produced TEM-1 + OXA-1. Forty-eight (94.1%) of the 51 isolates had the plasmid-mediated resistance gene flo ST to chloramphenicol and tetracycline. Combining enterobacterial repetitive intergenic consensus polymerase chain reaction (ERIC-PCR) and pulsed-field gel electrophoresis (PFGE), 16 distinct patterns were identified, among which patterns IA (35.3%) and IF (27.4%) were considered as epidemic patterns. The dendrogram obtained from S. Typhimurium pulsotypes allowed five clones (S1–S5) to be identified, with two prevalent clones comprising 47.8% (S2) and 27.3% (S4) of the isolates.

Molecular analysis of and identification of antibiotic resistance genes in clinical isolates of Salmonella typhi from India.

A representative sample of 21 Salmonella typhi strains isolated from cultures of blood from patients at the Christian Medical College and Hospital, Vellore, India, were tested for their susceptibilities to various antimicrobial agents. Eleven of the S. typhi strains possessed resistance to chloramphenicol (256 mg/liter), trimethoprim (64 mg/liter), and amoxicillin (>128 mg/liter), while four of the isolates were resistant to each of these agents except for amoxicillin. Six of the isolates were completely sensitive to all of the antimicrobial agents tested. All the S. typhi isolates were susceptible to cephalosporin agents, gentamicin, amoxicillin plus clavulanic acid, and imipenem. The antibiotic resistance determinants in each S. typhi isolate were encoded by one of four plasmid types. Plasmid-mediated antibiotic resistance genes were identified with specific probes in hybridization experiments; the genes responsible for chloramphenicol, trimethoprim, and ampicillin resistance were chloramphenicol acetyltransferase type I, dihydrofolate reductase type VII, and TEM-1 beta-lactamase, respectively. Pulsed-field gel electrophoresis analysis of XbaI-generated genomic restriction fragments identified a single distinct profile (18 DNA fragments) for all of the resistant isolates. In comparison, six profiles, different from each other and from the resistance profile, were recognized among the sensitive isolates. It appears that a single strain containing a plasmid conferring multidrug-resistance has emerged within the S. typhi bacterial population in Vellore and has been able to adapt to and survive the challenge of antibiotics as they are introduced into clinical medicine.

Evolution of resistance in microorganisms of human origin

Resistance to antimicrobials in bacteria results from either evolution of “new” DNA or from variation in existing DNA. Evidence suggests that new DNA did not originate since the use of antibiotics in medicine, but evolved long ago in soil bacteria. This evidence is based on functional and structural homologies of resistance proteins in human pathogens, and resistance proteins or physiological proteins of soil bacteria. Variation in existing DNA has been shown to comprise variations in structural or regulatory genes of the normal chromosome or mutations in already existing plasmid-mediated resistance genes modifying the resistance phenotype. The success of R-determinants in human pathogens was due to their horizontal spread by transformation, transduction and conjugation. Futhermore, transposition has enabled bacteria to efficiently distribute R-determinants between independent DNA-molecules. Since the genetic processes involved in the development of resistance are rare events, the selective pressure exerted by antibiotics has significantly contributed to the overall evolutionary picture. With few exceptions, experimental data about the role of antibiotic usage outside human medicine with respect to the resistance problem in human pathogens are missing. Epidemiological data about the occurrence of resistance in human pathogens seem to indicate that the major contributing factor to the problem we face today was the extensive use of antibiotics in medicine itself.