Case Study Evolution Antibiotic Resistant Bacteria In Meat

Food safety is important to prevent infection by contaminating foodborne microbes, and is becoming increasingly to prevent infection with drug-resistant foodborne microbes. The use of antibiotics in agriculture has increased antibiotic-resistant microbes in associated foodstuffs. Fortunately, governmental institutions like the Food and Drug Administration (FDA), the Center for Disease Control and Protection (CDC), and the U.S. Department of Agriculture (USDA) have many programs in place to prevent contamination as well as detect it when it does occur. A new study in Antimicrobial Agents and Chemotherapy reports how a routine surveillance program identified drug-resistant Salmonella contamination of grocery-store pork products.

AACJournal: Identification of plasmid-mediated quinolone resistance in Salmonella isolated from swine ceca and retail pork chops in the United States.

Map of where Salmonella containing one of two drug-resistance plasmid have been found in the United States. Source.

The National Antimicrobial Resistance Monitoring System (NARMS) surveillance program tracks changes in drug susceptibility in gut bacteria from ill people, retail meats, and food animals in the U.S. From 2002 to 2015, all Salmonella isolates were susceptible to ciprofloxacin, but 2013 to 2015 saw the emergence and increase of nonsusceptibility to 8% of isolates. To understand why, the research team led by first author Gregory Tyson and senior scientist Patrick McDermott sequenced the genome of 30 cipro-nonsusceptible isolates. This is how they happened to identify the plasmid-mediated quinolone resistance genes being carried by 27 of the 30 sequenced samples. Plasmids containing quinolone-resistance genes were isolated from swine cecal contents or retail pork samples in nine states (see right). 

Fluoroquinolone-resistant Salmonella are considered a serious threat to public health by the CDC. People sickened with fluoroquinolone-resistant enteric gram-negative bacteria have longer hospital stays and more complications. Additionally, plasmid-mediated drug resistance is more easily passed between bacterial species via horizontal gene transfer than genome-encoded resistance – meaning even people not sickened by these contaminating Salmonella could develop resistance in other members of their gut bacteria. 

Vegetarians might think they can rest easy, but a second report in the same Antimicrobial Agents and Chemotherapy issue described drug-resistant E. coli contamination among vegetables for sale in China. Similar to the study described above, the scientific team tested fresh vegetable samples for surveillance purposes, looking for the presence of extended-spectrum beta-lactamase (ESBL) production. Once identified, the ESBL-producing Enterobacteriaceae were screened for mcr-1, a gene that confers colistin resistance. Six of the 244 ESBL-producing isolates contained plasmid-borne mcr-1.

AACJournal: Emergence of mcr-1 in Raoultella ornithinolytica and Escherichia coli isolates from retail vegetables in China

The discovery of mcr-1 on fresh tomatoes and lettuce in China doesn’t mean only the Chinese need worry. Globalization means bacteria can travel between countries as easily as cargo ships or planes can, and our interconnected trading systems (including food trade) mean health systems of different countries can very quickly be similarly affected. The fact that these resistance determinants are all found on plasmids also bodes poorly for their containment. It’s worth noting that both reports found a second type of resistance after screening for a primary resistance unrelated to those reported, underscoring the problem of multiple drug resistance in our agriculture, foodstuffs, and other everyday interactions. The best recourse is to follow good food preparation techniques by thoroughly washing produce and cooking animal products to avoid exposure to these potentially dangerous pathogens. 

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