Foodborne Illnesses / Listeria /

Antimicrobial Resistance in Bacteria

Antimicrobial resistance is increasing in foodborne pathogens.

Antimicrobial resistance in bacteria is an emerging and increasing threat to human health. Physicians are increasingly aware that antimicrobial resistance is increasing in foodborne pathogens and that, as a result, patients who are prescribed antibiotics are at increased risk for acquiring antimicrobial-resistant foodborne infections. Indeed, “increased frequency of treatment failures for acute illness and increased severity of infection may be manifested by prolonged duration of illness, increased frequency of bloodstream infections, increased hospitalization, or increased mortality.”

L. monocytogenes, in general, is considered vulnerable to a wide range of antibiotics that have bactericidal effects against Gram-positive bacteria, including, tetracyclines, erythromycin, ampicillin, and gentamicin. However, most strains of L. monocytogenes exhibit native resistance to cefotaxime, cefepime, fosfomycin, oxacillin, and licosamides. Recently, antibiotic resistance among L. monocytogenes isolated from foods and the environment has increased, particularly for those antibiotics commonly used to treat listeriosis. Therefore, monitoring changes in the antibiotic resistance of L. monocytogenes due to the continuing emergence of resistant strains is needed.

The use of antimicrobial agents in the feed of food animals is estimated by the FDA to be over 100 million pounds per year. It is estimated that 36% to 70% of all antibiotics produced in the United States are used in a food animal feed or in prophylactic treatment to prevent animal disease. While antibiotics are commonly and broadly used in animal production, there is a lack of substantial research on the impacts to human health and benefits to the food industry. Additionally, there is high variability with policies around their use.

The unnecessary use of antibiotics in feed for food animals, on animals prophylactically to prevent disease, and in humans must be reduced. European countries have reduced the use of antibiotics in animal feed and have seen a corresponding reduction in antibiotic-resistant illnesses in humans.

Researchers have found that Listeria monocytogenes have varying resistance to certain antibiotics, depending on the strain. Many of these strains with resistance genes are isolated from food samples and food production areas. Listeria innocua, another similar species also found in food environments, more commonly displays antibiotic resistance and can be valuable for research. There is concern that L. monocytogenes strains will continue to acquire antibiotic resistance from shared genetic materials and environmental persistence, making severe illness harder to treat. This is especially concerning when the antibiotic resistance is seen in foods that are considered ready-to-eat (RTE) and thus less likely to have a control step before consumption. This research study discovered:

In the present study, Listeria isolates from RTE meat products displayed higher overall antimicrobial resistance (31.3%) than those from the environment (13.4%). Similarly, Kovacevic et al. indicated that antibiotic resistance was more commonly observed for L. monocytogenes in RTE foods than in processing environment samples, describing a co-selection phenomenon in which repeated exposure to sub-lethal concentrations of some antimicrobials (i.e. ciprofloxacin) may produce derivative strains possessing increased tolerance to the respective selective agent as well as increased tolerance to other antibiotics.