The Prevalence of an Inoculum Effect (InE) with Cefazolin (CFZ) and the Association with Certain blaZ Gene Types among Methicillin-Susceptible Staphylococcus aureus (MSSA) Isolates from Four Major Chicago Medical Centers

Background: CFZ offers a convenient dosing scheme, a favorable adverse event profile and is cost effective for MSSA infections. Recent clinical findings suggest treatment failure is no different between CFZ and oxacillin for deep-seated MSSA BSI. However, clinician preference for a semisynthetic penicillin over CFZ continues for high burden infections due to in vitro findings of certain blaZ genes (e.g. type A) known to hydrolyze CFZ inactive. This study aims to determine the local prevalence of an InE with CFZ and associated blaZ gene types among MSSA isolates collected from surrounding Chicagoland medical centers.

Methods: Four Chicago hospitals contributed MSSA isolates from any source (except urine/CSF) between 10/2014 and 2/2015. CFZ MICs were determined at 24 hours by broth microdilution method using standard inocula (SI) (5 x 10⁵ CFU/ml) and high inocula (HI) (5 x 10⁷ CFU/ml). The InE was defined as isolates with a ≥ 4-fold increase in CFZ MIC between SI and HI. A pronounced InE was represented by a ≥ 4-fold increase in MIC from SI to HI with a nonsusceptible CFZ MIC of ³16 mg/L at HI. PCR was used to amplify the blaZ gene. Followed by agarose gel electrophoresis and sequencing to determine the gene type.

Results:

InE, n (%)			blaZ Gene Types in Positive Samples, n (%)
MSSA Isolates	> 4-fold increase in CFZ MIC (SI to HI)	> 4-fold increase in CFZ MIC (SI to HI) and MIC ≥ 16 at HI*	MSSA Isolates	type A	type B	type C	type D
All Sites (n=306)	50 (16.3)	5 (1.6)	All Sites (n=198)**	61 (31)	25 (13)	110 (55)	2 (1)
Site 1 (n=80)	11 (13.8)	2 (2.5)	Site 1  (n=47) Pronounced InE (n=2)	16 (34) 1	2 (4)	27 (58)	2 (4) 1
Site 2 (n=68)	11 (16.2)	0 (0)	Site 2  (n=54)	15 (28)	9 (17)	30 (55)	0 (0)
Site 3 (n=81)	18 (22.2)	3 (3.8)	Site 3  (n=55) Pronounced InE (n=2)	14 (25) 1	11 (20)	30 (55) 1	0 (0)
Site 4 (n=77)	10 (13)	0 (0)	Site 4 (n=42)	16 (38)	3 (7)	23 (55)	0 (0)

*Pronounced InE

**Preliminary data. 198/310 MSSA isolates (64%) were blaZ positive.

Conclusion: In contrast to related studies, our MSSA isolates from varying infectious sites, present with a significantly lower prevalence of a pronounced InE with CFZ overall and with expression of the type A gene strain. The InE is an in vitro phenomenon. Other factors such as expression or dosage of the blaZ gene, geographical and seasonal influences, as well as site of infection may also influence this prevalence. Further research is needed.