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@joserrapa could you help explain the inoculum effect to me please?

@joserrapa my bad, i clicked on link and found out answer - basically MIC rises at higher inoculum / bacterial numbers. Weird!
Sounds like an antibiotic to finish a course of treatment rather than start one....

@gpollara

This is how I see it (disclosure: I’m clinician, not microbiologist)

Inoculum Effect (IE) is an in vitro phenomenon that occurs when antibiotic is tested against higher-than-normal colony counts.

In the case of cefazolin it appears occur with certain strains of MSSA that have a specific bla gene types (which cannot be ascertained in conventional antimicrobial susceptibility testing). Therefore IE it’s not supposed to affect peni-S S. aureus

@gpollara

So, from a clinician’s perspective:

1) Does IF (in vitro) have a clinical correlation (in vivo failure)? IDK

2) If yes, does IE overcome the benefits of cefazolin over antistaphylococcal penicillins?

To answer 2) we need to know the prevalence of IE-associated bla genes. In my country is approx 10% so cefazolin would be the drug of choice for most severe MSSA infections

3) Nevertheless, for high inoculum /Difficult infections (endocarditis /pneumonia) I start with cloxacillin