Salmonella is an enteric pathogen that causes quite a lot of foodborne illness. I learned there were several species of Salmonella bacteria of which the cause of typhoid fever was called Salmonella typi. Spread via food and water it used to kill a lot of people in the 19th and early 20th centuries. Nowadays all Salmonella bacteria are considered to be different subspecies (serovars) of just one species, Salmonella enterica. There are more than 2500 of them, of which several routinely infect humans. Salmonella enteritidis is the most common form of foodborne bacterial infection (NB: many foodborne infections are of unknown agent and many are probably viral). Since the 19th century we’ve cleaned up the water supplies with disinfection and filtration and improved the food supplies. We can even treat typhoid and some of the other salmonella infections with antibiotics. Typhoid fever responds well but is only a tiny fraction of salmonellosis these days. For non-typhoid salmonella gastroenteritis whether to treat with antibiotics is a matter of clinical judgment. Randomized trials don’t show a clear-cut benefit and even suggest antibiotic treatment may prolong bacterial shedding in the stools, but for serious illness or infants less than 2 months, people with significant co-morbidities like sickle cell disease or immune deficiencies, antibiotics are still used. The main types are fluoroquinolones (e.g., Cipro) or the new generation of cephalosporins (ceftrioxone). Unfortunately both classes of antibiotics are also used in industrial livestock production to increase growth of animals and that has caused worries that these huge operations are incubators for drug resistant organisms. A paper just being published in Foodborne Pathogens and Disease asked whether retail supermarket chickens might be a place where one could contract drug resistant salmonellosis. The answer seems to be, “maybe.” From the abstract:
Methods: We isolated Salmonella from raw chicken purchased from a randomly selected sample of retail outlets in central Pennsylvania during 2006-2007. Salmonella isolates from meat were compared, using pulsed-field gel electrophoresis, to isolates in the PulseNet database of Salmonella recovered from humans.
Results: Of 378 chicken meat samples, 84 (22%) contained Salmonella. Twenty-six (31%) of the Salmonella isolates were resistant to ≥3 antimicrobials and 18 (21%) were resistant to ceftiofur. All ceftiofur-resistant isolates exhibited reduced susceptibility (minimum inhibitory concentration >2μg/mL) to ceftriaxone and carried a blaCMY gene, as detected by polymerase chain reaction. Among the 28 Salmonella serovar Typhimurium isolates, 20 (71.4%) were resistant to ≥3 antimicrobials and 12 (42.9%) were resistant to ceftiofur. One ceftiofur-resistant Salmonella serovar Typhimurium poultry isolate exhibited a rare pulsed-field gel electrophoresis pattern indistinguishable from a human isolate in PulseNet; both isolates carried the blaCMY-2 gene. (M’ikanatha et al., Multidrug-Resistant Salmonella Isolates from Retail Chicken Meat Compared with Human Clinical Isolates, Foodborne Pathogens and Disease [onlinhe ahead of print; doi:10.1089/fpd.2009.0499, sub required]
In essence they determined a molecular fingerprint of salmonella isolated from supermarket chicken and compared it to those in a database of molecular fingerprints from salmonella isolated from people with salmonellosis. There was one exact match, but the sample was pretty small. Was chicken a source of that case? We don’t know. We only know that this rare salmonella serovar was found in both supermarket chicken and a human. It isn’t clear if they were even from the same geographic area. Of interest is that the match was for salmonella resistant to the new generation of cefalosporins. Make of that what you will.
About a fifth of the supermarket chicken samples contained salmonella. That doesn’t mean you would get sick from eating the chicken if it was properly prepared and cooked, since cooking kills salmonella. On the other hand, it would be possible to cross-contaminate foods that are eaten raw, say by cutting salad on a cutting board that was not properly washed after being used to cut up the chicken or by unwashed hands that handled the raw meat and then the salad. Sometimes juice from the raw meat drips onto food eaten raw. Lots of possibilities, so don’t count on cooking alone to protect you.
Of the salmonella that was there, almost a third were resistant to three or more antibiotics and a fifth to the third generation cephalosporins. This included the one that matched the fingerprint of the human isolate in CDC’s Foodnet surveillance system.
If large commercial poultry operations are breeding grounds for drug resistant salmonella, the bugs still have to get out and into the community. This small study shows that they make it all the way to the supermarket refrigerator case as one possible route.