Tag Archives: antibiotic resistance

Salmonella and misinformation?

Every now and then we hear news of potential outbreaks of Salmonella linked to food products. If you have followed the news you will note a recent outbreak linked to raw chicken (an excellent article by Maryn McKenna is here). Many people, including myself, read these articles and then read comments posted below the articles. Comments are great because it connects the public with the author and allows for free discussion. However, I was amazed at the misinformation that is also spread via these forums. I thought it would be worthwhile to revisit Salmonella basics related to 1) cooking and 2) antimicrobial resistance.

1) Cooking. Salmonella are basically E. coli that have evolved to acquire the ability to become intracellular pathogens and cause both diarrhea and systemic disease. However, the basic cellular components between these bacteria are still the same. They are both Gram-negative bacterial cells that do not form spores. Salmonella also do not release toxins like Clostridium botulinum (the cause of botulism) that can kill you even when the bacteria are dead. Therefore, from a foodborne illness standpoint, heat is an extremely effective way to eliminate Salmonella. Cooking meat to an internal temperature of 165 degrees F will ensure that you have zero risk of getting a Salmonella infection from the food itself.

The mistake most people make, though, is in food preparation. The bacteria can be present on raw meat (you should always assume it is). Therefore, separating the “raw” food prep area from the cooked prep area, and washing everything including your hands immediately after dealing with raw product, will substantially reduce your chances of spreading live bacteria to somewhere where they can be ingested. Keep in mind that household pets can also be carriers of Salmonella! If you give your dog raw meat, he/she can then pass the live cells on to the kids that the dog licks five minutes later. Following these basic practices reduces your chances of getting Salmonella from meat at home to very little / none.

It’s also worth noting that regulations are in place to keep Salmonella counts on retail raw meats down. A division of USDA called FSIS monitors the amount of Salmonella-positive samples from live birds and raw meat coming from large commercial broiler and turkey operations. One might ask, “why allow any Salmonella in raw meat products?” A simple answer is that it is currently impossible to eliminate Salmonella from live poultry operations. Birds are an ideal niche for Salmonella because they can carry many of the human-relevant serovars of Salmonella asymptomatically, meaning birds carry the Salmonella in their guts and display no signs of illness. One might also reason that buying organic chicken and turkey is safer than conventionally-raised birds. In fact, there is no difference between these two types of meat, both contain Salmonella along with Campylobacter (example study here). From the time that humans have started raising chickens and turkeys, there has been Salmonella – and that is not going to change anytime soon.

The bigger danger is eating raw vegetable and other foods contaminated with Salmonella. The extensive list of Salmonella outbreaks on the CDC website include foods such as mangoes, cantaloupe, sprouts, nuts, and tomatoes. These present a much greater risk because you may not be cooking these food products. It is advisable to thoroughly wash any raw vegetables and even soak them if possible to reduce your chances of getting infected. Fortunately the presence of Salmonella on these types of food is much less common than on raw meats.

2) Antimicrobial resistance. A final point is that there is major concern about antimicrobial resistance in Salmonella. This is a valid and true concern, as it is evident that Salmonella along with many other pathogens are evolving to become resistant to many therapeutic options available for disease treatment. Fortunately, if you get a Salmonella infection it is very unlikely that you will require antibiotic treatment unless you are infant/elderly. This doesn’t discount though that many Salmonella are now resistant to the most commonly used drugs to treat these infections, and it presents a great risk to immunocompromised populations. Before we jump to conclusions about banning antibiotics from agriculture, though, we probably need to look in the mirror. I have no doubts that all antibiotic use in some way contributes to the emergence of antimicrobial resistant bacteria. However, there are many necessary scientific studies needed to get at the actual risk posed by different applications of antibiotics. For example, which is worse: treating with a very low concentration of an antibiotic not used in human disease therapy for extended time, or hitting with a high dose of an essential human antibiotic for short duration? There is a wealth of literature out there, but even among scientists there still exists debate about this topic. Before jumping to any legislative conclusions, we need more science and data to support these decisions.

CRE: latest catch phrase in infectious disease

You have probably heard in the news about CRE, or carbapenem resistant Enterobacteriaceae. The MN Department of Health has a very nice page on CRE here. CRE has certainly gotten a lot of hype about being the next major “superbug” – and this is with good reason. But I wonder if the media going to the well one too many times for the “superbug” catchphrase is going to hurt us all in the long run? Is CRE really a superbug? It is important to note that CRE actually represent any Enterobacteriaceae that are resistant to carbapenems. This can encompass a wide variety of bacteria causing different diseases. The major player is Klebsiella pneumoniae, which is most commonly associated with urinary tract infection and resulting sepsis. This type of infection, if untreatable with antibiotics, can certainly be deadly. This is the worst case scenario, and there are a range of infections in between that can involve CRE and may or may not require antibiotic treatment. I think there are some important points to consider when discussing CRE.

CREs are mostly created through the conjugative transfer of plasmids that encode a carbapenemase. These can include KPC, NDM, and VIM. The latter two have emerged in the US only recently, but KPC in K. pneumoniae has been around for a while. The dangers of these genes being carried on plasmids is that they can move between bacteria via bacterial conjugation, and these plasmids can carry resistance to additional antibiotics. What is not well understood at this point is the extent to which conjugation occurs and precisely where these plasmids tend to spread. Plasmids encoding KPC, NDM, and VIM are diverse and confer different arrays of resistance phenotypes. It is clear that selective pressures are driving the integration of these genes into a variety of plasmid types, but it is less clear why certain plasmid types (such as IncA/C) are able to carry these genes and so many other resistance genes at the same time, with little cost to the bacterial host. One thing is clear – our labeling of these plasmids as “promiscuous” (like I have done many times) needs to be done with caution. If plasmids such as IncA/C were as promiscuous as I originally thought they were, they would have easily swept E. coli and Salmonella populations by now and we would be experiencing a pandemic of MDR E. coli and Salmonella carrying IncA/C plasmids (and CRE bacteria). This is not the case and probably will not ever be the case. There is something that limits the dissemination of these plasmids among E. coli and Salmonella. Could it be a quorum sensing mechanism that limits the conjugation of this plasmid? Maybe. But my main point is that CRE is certainly concerning. It is a fascinating research topic. However, I am hesitant to call CRE a superbug because my view of a superbug is a potential global killer. Let’s hope that CRE does not reach that status.