Antimicrobial resistance and R factors in Salmonella isolated from humans and animals in Georgia and South Carolina

Emergence and spread in Taiwan of multidrug-resistant serotypes of nontyphoidal Salmonella

PURPOSE

Antimicrobial resistance of nontyphoidal Salmonella (NTS) is a threat to public health worldwide.

METHODS

A study on longitudinally collected NTS isolates from a medical center in Taiwan from 2011 to 2019 was undertaken. The multidrug resistance (MDR) and extensively drug resistance (XDR) phenotypes were determined according to internationally used definitions. Molecular serotyping was performed on the resistant NTS.

RESULTS

Notably 16.1% (870/5412) of the isolates were MDR, while XDR accounted for 2.1% (111/5412). Both MDR and XDR NTS have increased significantly from 2011 to 2019, especially from 2015 to 2017 (MDR from 9.6% in 2015 to 23.1% 2017; XDR from 1.4% in 2016 to 4.7% in 2017). S. Anatum was the commonest NTS serotype expressing MDR and XDR, in 256/559 (45.8%) and 81/111 (73.0%) of the isolates, respectively, followed by S. Typhimurium and S. Goldcoast. Children < 18 years old contributed to 69.0% of all MDR cases and 64.0% of all XDR cases; majority of them aged less than 5 years.

CONCLUSIONS

Increasing MDR and XDR NTS is a threat to public health. MDR and XDR NTS usually caused gastroenteritis in children < 5 years old. Multiple NTS serotypes expressing MDR and XDR indicate multiple food vehicles involved in the transmission. Proper food hygiene practice should never be over-reinforced.

Resistance to the nitroheterocyclic drugs

The nitroheterocyclic drugs have been available since the early 1960's for the treatment of anaerobic protozoa. The application of these drugs has widened since then and they are presently used to treat anaerobic pathogenic bacteria and protozoa. The activity of the nitroheterocyclic drugs depends on the all-important nitro group attached to the imidazole or furan ring. Although the nitro radicals, generated by reduction of the parent drugs, are similar for both families of nitroheterocyclics, the nitroimidazoles and the nitrofurans, the electron potential of each is different and thus the mechanism of action depends on different pathways. The nitroimidazoles depend on reduction by ferredoxin or flavodoxin. The nitrofurans require nitroreductase activity, but the natural substrate of these enzymes has not been identified. Increased use of nitroheterocyclic drugs, in response to drug resistance to other commonly used antibiotics, has in turn resulted in drug resistance to a number of nitroheterocyclic drugs. Bacteroides strains and other bacteria, including Helicobacter, have developed resistance. Among the protozoa, Trichomonas has developed resistance to metronidazole via a number of mechanisms, especially a decrease in drug reduction, as a result of alterations in the electron transport pathways. Resistance to both types of nitroheterocyclic drugs has been reported in Giardia. Although resistance to these drugs is not widespread, their increased use world-wide as a prophylaxis and in chemotherapy will inevitably result in increased resistance in organisms commonly found in asymptomatic infections, including Trichomonas, Giardia and Entamoeba. However, the variety of substitutions which can be attached to the ring structures has led to a great variety of drugs being synthesised, some of which are many-fold more active than the commonly prescribed nitroheterocyclics. With careful administration of currently available drugs and continued interest in synthesising more active compounds, we can optimistically expect to have useful nitroheterocyclic drugs available for some time.

Drug-resistant Salmonella from animals fed antimicrobials

It has been difficult to document the postulated sequence of events that begins with the selection of drug-resistant organisms in animals fed subtherapeutic amounts of antimicrobials and ends with clinically important infections in human beings. In early 1983 we identified 18 persons in four Midwestern states who were infected with Salmonella newport that was resistant to ampicillin, carbenicillin, and tetracycline and characterized by a 38-kilobase R plasmid. Twelve of these patients had been taking penicillin derivatives for medical problems other than diarrhea in the 24 to 48 hours before the onset of salmonellosis. Eleven patients were hospitalized for salmonellosis for an average of eight days, and one had a fatal nosocomial infection. We compared plasmid profiles of all human (six-state area) and animal (United States) S. newport isolates over an 18-month period and examined selected records of meat distribution. The results indicated that the patients had been infected before they took antimicrobials, by eating hamburger originating from South Dakota beef cattle fed subtherapeutic chlortetracycline for growth promotion. This study demonstrates that antimicrobial-resistant organisms of animal origin cause serious human illness, and emphasizes the need for more prudent use of antimicrobials in both human beings and animals.

Bactericidal activities of chloramphenicol and eleven other antibiotics against Salmonella spp

The bactericidal activity of chloramphenicol against 27 strains of Salmonella typhi and 33 strains of S. enteritidis was compared with those of 11 other antibiotics. The geometric mean bactericidal concentrations of chloramphenicol against susceptible strains (36.10 and 43.13 micrograms/ml for S. typhi and S. enteritidis, respectively) far exceeded those of the other 11 antibiotics, with cephalothin having the next highest values (2.67 and 8.66 micrograms/ml) and moxalactam (0.09 and 0.28 micrograms/ml), cefotaxime (0.08 and 0.28 micrograms/ml), ceftriaxone (0.07 and 0.16 micrograms/ml), norfloxacin (0.06 and 0.10 micrograms/ml), and aztreonam (0.05 and 0.20 micrograms/ml) having the lowest values. The results for imipenem (0.24 and 0.81 micrograms/ml) and ceftazidime (0.22 and 0.75 micrograms/ml) were lower than those noted for trimethoprim-sulfamethoxazole (1.20 and 5.56 micrograms/ml), cefamandole (0.62 and 3.29 micrograms/ml), and ampicillin (0.55 and 2.78 micrograms/ml). The MBC of chloramphenicol for some isolates decreased with increased incubation times such that the proportion of susceptible isolates killed by chloramphenicol at concentrations within achievable levels in blood increased from 10% after 24 h to 26% after 48 h of incubation. Although the MBC of the other 11 antibiotics for some isolates were also lowered by prolonged incubation, all 24-h values were within achievable levels in blood. The data indicate that chloramphenicol is not uniformly bacteriostatic against S. typhi and S. enteritidis. The in vivo significance of demonstrating delayed killing by chloramphenicol is, however, uncertain.

Plasmid-mediated resistance to trimethoprim-sulfamethoxazole in Salmonella krefeld strains isolated in the United States

Two types of transmissible plasmids which encode resistance to trimethoprim-sulfamethoxazole were found among five animal isolates of Salmonella krefeld. This is the first report of plasmid-mediated resistance to trimethoprim-sulfamethoxazole in Salmonella strains isolated in the United States.

Antibiotic resistance and R-plasmids in food chain Salmonella: evidence of plasmid relatedness

A large number of strains (1,783) belonging to 15 Salmonella serovars isolated, in Canada, from the three major links of the human food chain were screened for multiple antibiotic resistance and the presence of R-plasmids. Multiresistant strains occurred among animal feed, livestock, and human isolates at frequencies of 4, 22, and 14%, respectively. Conjugation analysis revealed that 58% of the isolates from feeds, 87% of those from livestock, and 89% of the human strains carried all or part of their resistance determinants extrachromosomally on R-plasmids. Conjugative plasmids representing nine different incompatibility groups were detected, with the Inc I alpha group being predominant. Within the limits of the parameters measured, certain of these plasmids show a degree of relatedness suggestive of a common ancestry.