8. Unusual Organisms

 Application of the CDS to Unusual organisms

It is simply not possible to calibrate formally the CDS for every organism that could be associated with an infective process. In the first place gathering a sufficient number of unusual isolates can be extremely difficult and secondly, we do not have the resources to carry out the procedure on endless number of species.  The advice given below is based on less formal testing in the CDS laboratory of these species. When requested, the interpretation of results of testing of organisms not calibrated by the CDS may be extrapolated, with caution, from that of organisms with similar characteristics and growth requirements. The report should include a comment to indicate it is a provisional result and if necessary the susceptibilities may need to be confirmed by a quantitative technique. The species where this applies are grouped below.

Aerococcus sp., Gemella sp., Granulicatella sp., Abiotrophia defectiva: For these catalase negative, Gram positive cocci, the interpretation of AST results is extrapolated from those of streptococci. As with streptococci they are tested on Blood Sensitest Agar in an atmosphere of 5% CO2 and with the antibiotic discs calibrated for Streptococcus sp. If the organism has an inhibitory zone < 6mm with P 0.5 u and an inhibitory zone ≥ 6mm with AMP 5 µg, the organism is reported as having a decreased susceptibility to benzylpenicillin, amoxycillin, ampicillin with an MIC between 0.25 and 2mg/L. Aerococcus sp. are generally susceptible to benzylpenicillin and resistant to aminoglycosides. Aerococcus sp. can also be tested against ciprofloxacin 2.5 µg disc calibrated for Staphylococcus sp.

Pediococcus sp., Leuconostoc sp.: These catalase negative, Gram positive cocci species are inherently resistant to vancomycin and teicoplanin. Interpretation of testing of these species is the same as that for streptococci. They are tested on Blood Sensitest Agar, in an atmosphere of 5% CO2 and with the antibiotic discs calibrated for Streptococcus sp. As with streptococci, if the organism has an inhibitory zone < 6mm with P 0.5 u and an inhibitory zone ≥ 6mm with AMP 5 µg, the organism is reported as having a decreased susceptibility to benzylpenicillin, amoxicillin and ampicillin with an MIC between 0.25 and 2mg/L.

Coryneform group (Arcanobacterium sp., Dermabacter sp., and Rothia sp.): These species are tested on Blood Sensitest Agar, in an atmosphere of 5% CO2 and with the antibiotic discs calibrated for Corynebacterium sp. and interpretation of the results is the same as that with Corynebacterium sp. Interpretation of AST results is extrapolated from those of Corynebacterium sp. If the organism has an inhibitory zone of < 6mm with P 0.5u and an inhibitory zone of ≥ 6mm with AMP 5 µg, the organism is reported as having a decreased susceptibility to benzylpenicillin, amoxycillin, ampicillin with an MIC between 0.25 and 2mg/L.

Bacillus sp:  The interpretation of results may be extrapolated from those of Corynebacterium sp. However, unlike Corynebacterium sp., some members of Bacillus species may produce a β-lactamase. Therefore, we recommend only the use of a P 0.5 u disc for the testing and reporting of the susceptibility to benzylpenicillin, amoxycillin and ampicillin. Do not test Bacillus sp. against AMP 5 µg disc as this disc may give false susceptibility with β‑lactamase positive isolates.

 

Non-fastidious, non-Enterobacteriaceae Gram Negative Group:  Including Achromobacter sp., Chromobacterium sp. and B. bronchiseptica can be tested as outlined in the Pseudomonas, Burkholderia & Chryseobacterium species (Table 11.1.b.).

Actinobacillus sp., Aggregatibacter sp., Cardiobacterium, Capnocytophaga sp., Eikenella, Kingella sp. and other fastidious Gram negative organisms: The interpretation of AST results is extrapolated from those of Pasteurella sp. They are tested on Blood Sensitest Agar, in an atmosphere of 5% CO2 using the discs calibrated for Pasteurella sp. Slow growing organisms may require a suspension adjusted to an equivalent 0.5 McFarland standard instead of the standard inoculum of 107 cfu/mL.