The High Prevalence of Multidrug Resistant Salmonella in Food—An Important Public Health Concern

Abstract & Commentary

Synopsis: Twenty percent of samples of ground meats from Washington DC area stores contained salmonella. Eighty-four percent of the isolates were resistant to at least 1 antibiotic.

Source: White DG, et al. N Engl J Med. 2001;345:1147-1154.

Salmonella is a leading cause of foodborne illness. The emergence of antimicrobial-resistant salmonella is associated with the use of antibiotics in animals raised for food; resistant bacteria can be transmitted to humans through foods. In this study, salmonella was isolated from samples of ground chicken, turkey, beef, and pork purchased at 3 supermarkets. The isolates were characterized by serotyping, antimicrobial-susceptibility testing, phage typing, and pulsed-field gel electrophoresis.

Of 200 meat samples, 41 (20%) contained salmonella, with a total of 13 serotypes. Eighty-four percent of the isolates were resistant to at least 1 antibiotic, and 53% were resistant to at least 3 antibiotics. Sixteen percent were resistant to ceftriaxone, the drug of choice for treating salmonellosis in children. Bacteriophage typing identified 4 isolates of Salmonella enterica serotype typhimurium definitive type 104 (DT104), 1 of DT104b, and 2 of 208. Five isolates of S enterica serotype agona had resistance to 9 antibiotics and 2 of the isolates of serotype typhimurium DT208 were resistant to 12 antibiotics. Electrophoretic patterns of DNA which were indistinguishable from each other were found in isolates from different meat samples and different stores. Eighteen isolates, representing 4 serotypes, had integrons with genes conferring resistance to aminoglycosides, sulfonamides, trimethoprim, and beta-lactams such as ceftriaxone.

Resistant strains of salmonella are common in retail ground meats. These findings provide support for the adoption of guidelines for the prudent use of antibiotics in food animals and for a reduction in the number of pathogens present on farms and in slaughterhouses. National surveillance for antimicrobial-resistant salmonella should be extended to include retail meats.

Comment by Ralph R. Hall, MD, FACP

This study, in itself, is cause for concern but added to this information are the data from other recent studies.

A second study by McDonald and colleagues in the same issue found that 17% of chickens from supermarkets in 4 states had strains of Enterococcus faecium that were resistant to a new antimicrobial agent, quinupristin-dalfopristin, that has recently been approved for use in humans.1 This is apparently related to the use of a similar antibiotic used in chicken feed.

A third study in the same journal examined glycopeptide-resistant and streptogramin-resistant strains of E faecium, isolated from chicken obtained at a grocery store.2 A previously reported study by Manges and colleagues suggests that antibiotic-resistant strains of Escherichia coli responsible for an epidemic of urinary tract infections may be the result of contaminated food.3

Two excellent editorials, by Stamm4 and Gorbach5 point out that a "multidisciplinary and worldwide" approach is needed to solve this problem. The antibiotics given to food animals for infections and growth promotion need more effective regulation and perhaps should only be given under the direction of veterinarians. Gorbach suggests that subtherapeutic use of these agents be banned since there is evidence that improvements in animal husbandry, the quality of feed, and hygiene can prevent any economic losses for the animal food industry.

Gorbach sites a report by The Union of Concerned Scientists that estimated that each year 24.6 million lbs of antimicrobials are given to animals for nontherapeutic purposes, 2 million lbs are given to animals for therapy, and that 3 million lbs are given to humans.6

White and colleagues point out that ceftriaxone is commonly used to treat children with salmonella infections because of its favorable pharmacologic properties and low prevalence of resistance. Ciprofloxin, a fluoroquinolone, and trimethaprim-sulfamethoxazole are also used for therapy in children with serious salmonellosis. The recent prophylactic use of ciprofloxin for the prevention of anthrax will undoubtedly add to our problems of selecting treatment for these serious infections.

References

1. McDonald LC, et al. N Engl J Med. 2001;345:1155-1160.

2. Sorensen TL, et al. N Engl J Med. 2001;345:1161-1166.

3. Manges AR, et al. N Engl J Med. 2001;345:1007-1013.

4. Stamm WE. N Engl J Med. 2001;345:1055-1057.

5. Gorbach SL. N Engl J Med. 2001;345:1202-1203.

6. Mellon M, et al. Union of Concerned Scientists; Cambridge, Mass: 2001

Dr. Hall, Emeritus Professor of Medicine, University of Missouri-Kansas City School of Medicine, is Associate Editor of Internal Medicine Alert.