I ran an antibiotic susceptibility test using Vitek2 on Klebsiella pneumoniae. The result shown it was sensitive to ciprofloxacin and levofloxacin (quinolones drugs) judging from MIC values. However, phenotypic interpretation revealed it was partially quinolones resistance. How does both results correlated? Please enlighten me. Thank you.
Hello Heng, I think you need to compare your results with CLSI standards. However, you should determine the concentration of the disk if performed or do allow excessive time of of your susceptibility testing before incubation or at what hour do you notice such appearance of growth. The time of exposure of the antibacterial agents and concentration against the organism needs to be determined. Please go through the uploaded article, though it is on medicinal plants but can offer you insight to your question.
Article In-Vitro Antibacterial Time-Kill Assay Of Phyllanthus Amarus...
Hello, I might understood what you mean. Quite commonly in the last years, Enterobacteriaceae have become resistant to Ciprofloxacin and sometimes Levofloxacin too. In our hospital, for instance, we have the 43% of resistance to ciprofloxacin. Is it possible that, according to the complite phenotype, the instrument, choose to modify the interpretation of the MIC?
Hello
The most probable is that your isolates will have a fluoroquinolones (FQ) MIC of around 0.25 - 0.5 mg/L , which is considered as susceptible. However, with these MICs values, the isolates usually possess a mutation in gyrA and/or possess any transferable mechanisms of quinolone resistance (TMQR).
I suppose that when you say "phenotypic interpretation revealed it was partially quinolones resistance" you say that your results with disk difussion indicate that your isolates are intermediate resistance. If I am sure in that regarding MIC values this is not surprising, as you are just in the border of the breakpoint, and then the halo diameter will be reduced and may fall within the intermediate interpretation, in special if the inoculum is so high.
Although the current expansión of TMQR may result in the presence of FQ in absence of nalidixic acid resistance, I suggest that use a disk of Nal. If your isolates are Nal-resistant, probably the presence of a mutation in gyrA is the most feasible scenario.
In any case presence of minor discrepances (susceptible / intermediate) using different techniques it is not imposible (J. Clin. Microbiol. September 2011 vol. 49 no. 9 3343-3345)
I dont know, but I think is referred to those bacteria called "moderately resistant", or "intermediate" in accordance to CLSI guides
I do not understand some points in your question which I have given below with possible explanations:
1. Are you mixing up the words intermediate & partial resistance? The textbook Antibiotic & Chemotherapy by Finch et al states in chapter 3 “The clinically intermediate (EUCAST) or intermediate (CLSI) category is used for bacteria with an MIC that lies between the breakpoints for clinically susceptible and clinically resistant. These strains are inhibited by concentrations of the antimicrobial that are close to either the usually or the maximally achievable blood or tissue level and for which the therapeutic response rate is less predictable than for infection with susceptible strains. This category also provides a technical buffer zone that should limit the probability of misclassification of bacteria in susceptible or resistant categories”. Does that answer your question?
2. Are you confusing partial & complete cross resistance described for Staphylococcus or S. pneumoniae with K. pneumoniae in EUCAST when interpreting results for quinolones ? Well these terms are used to explain sequential development of resistance by point mutations in gyrA & parC. Hence , if a tested S. pneumoniae is resistant to ofloxacin or ciprofloxacin, then test the organism for levofloxacin or moxifloxacin- if it turns out to be sensitive then the organism does not exhibit complete or partial cross resistance to quinolones, however if it is resistant to either levofloxacin or moxifloxacin then it may be completely/ partially resistant to quinolones. Attaching a pdf that explains this ! !
3. MIC is also a phenotypic test, as is disc diffusion & Etest? So, what do you mean by phenotypic test? Genotypic tests usually will not give any information whether the organism exhibits intermediate resistance. Please elaborate your question
Thanks for asking this, it has cleared a lot of my own doubts while trying to answer this question. Regards Vinay
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