How to read the cumulative antibiogram and its usefulness

Here are example antibiograms for Hunter New England (NSW). These are constructed by assessing the % susceptible for specific bugs and drugs, taking only the first isolate of a species per person per annum.  Where is it possible extrapolate susceptibility rather than test or we know a particular species is intrinsically susceptible, we enter “S”. Conversely, species that are intrinsically resistant to an antibiotic (e.g. Gram negatives and vancomycin) are marked as “R”.  In either situation, there is no rationale for actually testing that antibiotic against the species.

Here is a handy single page explanation of the different bacterial species groups for which antibiogram data is relevant. It also goes over what wild-type and acquired resistance characters may be expected.  Bacterial species grouping and AST antibiograms.  Abbreviations: FOX=cefoxitin, CMP=chloramphenicol, VAN=vancomycin, GEN=gentamicin, MTZ=metronidazole etc.

Provided that the antibiogram data are correct (an issue to do with testing QC and isolate selection), antibiograms help clinicians to construct treatment guidelines that reflect the local susceptibilities of key pathogens.

  1. For empirical treatment of patients with severe sepsis (patient is critically ill, bacterial sepsis is a part of the differential diagnosis), it is important to know what broad spectrum antibiotics are required to cover the likely pathogens as every hour of delay in appropriate treatment increases the case mortality rate!  Looking at the urine isolate antibiogram, once can design a regimen that will be suitable to cover the Gram negative pathogens – this may involve a combination of agents to achieve the coverage. Gentamicin is usually the backbone, given its rapidly bactericidal action and its excretion to the renal tract.  For severe sepsis due to disseminated  bacterial infection or infection at other sites, it is important to reference the non-urine antibiogram or even an antibiogram for bloodstream isolates before designing the empirical regimen.
  2. For treatment of infection in a particular site,  the susceptibility of likely pathogens in that site can be examined and then the  preferred antibiotic selected – for instance skin and soft tissue infection – Staph. aureus (including MRSA) and betahaemolytic streptococci require coverage- in PNG, cotrimoxazole represents a good choice based on recent antibiogram data.
  3. Treatment of certain fastidious pathogens such as N. gonorrhoeae, Salmonella Typhi , Vibrio cholerae or Shigella species, may need to reference data from expert reviews, research studies or the local reference laboratory (some PNG related references here- Shigella isolates commonly resistant to ampicillin, tetracycline, co-trimoxazole and chloramphenicol).


About mdjkf

Microbiologist and Infectious Diseases Physician
This entry was posted in Antimicrobial susceptibility testing, Med Micro topics-advanced and tagged , . Bookmark the permalink.

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