I'm doing this project to identify MRSA. In my antibiogram there are some samples that are showing resistance to oxacillin but not in cefoxitin. What might be the interpretation? Is it because oxacillin resistant isolates are having mecC gene or because of its heteroresistamce or it's just a false positive result?
Heteroresistance means that a population consists of bacteria with (slight) variations (or a range) in susceptibility. A part of the population might be in the susceptible range while another part is in the intermediate or resistant range. And this is the difference: if your isolate is within one of the three categories (i.e. susceptible, intermediate or resistant) the whole population is either susceptible, intermediate or resistant. The crux is that if you test with systems like Phoenix or Vitek, you are not able to detect heteroresistance. For this, you need disc diffusion. A heteroresistant population shows up with faint growth of the same isolate within the inhibition zone.
However, your MRSA conflict sounds being different. According to your description, you have isolates, which are oxacillin-resistant but cefoxitin susceptible. This is not a result of heteroresistance but due to a specific resistance mechanism. mecC MRSA typically show cefoxitin resistance but oxacillin susceptibility (quite the opposite of what you are observing). If the oxacillin MIC is between 1 and 8 mg/l, a possible explanation could by the expression/hyperproduction of a beta lactamase (borderline oxacillin-resistant S. aureus, BORSA).
You may check the recommended links for BORSA and heteroresistance.
https://www.ncbi.nlm.nih.gov/pubmed/28893360
http://cmr.asm.org/content/28/1/191.long
Answer above is very well written..I am adding a simple version.heteroresistance is resistance in a clone of cells among entire population of a species while intermediate resistance is about whole isolate whose zone size is coming between susceptible n resistant one..heteroresistance can be measured by genotypic methods ..
Well presented answers. Just to add to that. Heteroresistance is the different levels of resistance to an antibiotic by sub-populations of the species. I had similar results where some S. aureus isolates were resistance to oxacillin but sensitive to cefoxitin and they were subjected to PCR detection of mecC and MecA. None was positive for these genes. However, those that were resistant to cefoxitin were positive for MecA. Resistance to only oxacillin but sensitive to cefoxitin could be due to other mechanisms of resistance to only that antibiotic. PCR will help detect if it is MRSA or not.
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