Drug Resistance in Food: Chicken, Shrimp, Even Lettuce (ICAAC 4)

A final post from the ICAAC meeting, which concluded at one end of the Moscone Center in San Francisco Wednesday just as the Apple iPhone 5 launch was beginning at the building’s other end. (Definitely a crossing of geek streams.)

There’s far too much going on at a meeting like this to cover everything. So what emerges, as journalists move around the session rooms and exhibit floors, are stories regarding whatever caught a reporter’s eye based on his or her existing interests and news sense.

What caught my eye was a lot of research into foodborne illness, and particularly into the possibility of food being a reservoir for antibiotic resistance (which, constant readers will know, is something I’m interested in).

So, a quick run through some of the many presentations on this topic:

First, a team from the Centers for Disease Control and Prevention, led by epidemiologist Jared Reynolds, tracked the emergence of resistance to the drug ceftriaxone in Salmonella isolates collected from human patients by the federal National Antimicrobial Resistance Monitoring System (NARMS). When you think of Salmonella infection, you might think of gut disturbance that runs its course after a few days. But Salmonella can also cause very serious invasive disease, so the ability to shut an infection down with antibiotics is crucial. Ceftriaxone is one of the few remaining drugs that performs reliably, and yet ceftriaxone resistance in Salmonella has been increasing, making cures more difficult to achieve.

The group looked for evidence of ceftriaxone resistance between 1996 and 2010, and noted that it has been rising, from 0.2 percent of isolates collected in the NARMS program in 1996 to 2.8 percent in 2010. Some of the isolates were extremely drug-resistant, not responding to seven different families of antibiotics, including the one that ceftriaxone belongs to. Something else odd: The broad category of Salmonella has a number of serotypes within it, and different serotypes are associated with different foods, such as Salmonella Newport with beef, for example, and Salmonella Heidelberg with chicken. Resistance in Salmonella Newport is trending down after a peak between 1998 and 2002, the group noted. But the incidence of resistance in Salmonella Heidelberg has been rising rapidly since 2008, and no one can yet say why.


A team from the European Union performed similar analyses using data from the European Antimicrobial Susceptibility Surveillance in Animals (EASSA) system, a surveillance project that is similar to NARMS in the U.S. They looked at antibiotic resistance in E. coli and Salmonella found in cattle, pigs and chickens. The study covered 10 countries, and the results were variable across countries. Generally, though, the team found that bacteria taken from the animals at slaughter exhibited high rates of resistance to older antibiotics, and diminished susceptibility, but not yet frank resistance, to newer drugs. Oddly, the researchers noted, resistance in chicken and pigs to the drug chloramphenicol was high (16 percent of samples) despite the drug having been banned for years.

Pushing the research objective from animals as they are being slaughtered to the meat those animals become, a team from Portugal updated an analysis of retail beef, chicken and pork that they did 7 years ago. They found that 87 percent of their samples carried resistance to at least one antibiotic, with tetracycline being the most common. A nationwide program aimed at controlling Salmonella succeeded in reducing incidence of one important subtype, Enteriditis — but, the researchers said, it was replaced by the subtype Typhimurium, which tends to be more resistant.

A team from Australia tackled the difficult question of why poultry in that country carry E. coli that exhibit resistance to the drug class fluoroquinolones (such as Cipro), given that Australia has never permitted fluoroquinolone use in poultry raising. In a small study, they found that 30 percent of isolates which carried fluoroquinolone resistance also exhibited resistance to the drugs amoxicillin, gentamicin, tetracycline and trimethoprim-sulfamethoxazole, which are allowed in poultry there. They found that the resistance genes all resided on the same genetic mobile element. They hypothesize that use of the legal poultry drugs is driving the emergence of resistance both to the legal drugs and to fluoroquinolones. The fluoroquinolone ban was intended to protect the drug for use in human infections, but the the existence of this “co-selection” suggests the drugs’ usefulness could be undermined nonetheless.

A multi-national team from Japan and Thailand explored the effect of antibiotics on shrimp farming by examining E. coli collected from farm waters and from shrimp taken to market. More than 17 percent of the bacterial samples were resistant to tetracycline, and many of the samples were multi-drug resistant.

Finally, researchers from Detroit and Rochester, Michigan, wanted to investigate whether antibiotic-resistant bacteria on food were creating human health problems. They examined retail lettuce for bacteria known to cause human disease and checked whether those bacteria were drug-resistant. They were, in fact, multi-drug resistant — and they were almost identical to resistant strains of the same bacteria that were causing infections in patients in two Michigan health care systems.

Flickr/Robert Couse-Baker/CC


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