Evaluation of presence, predictors, causes and clinical results of inoculum effect against cefazoline among methicillin-susceptible staphylococcus aureus strains isolated from bloodstream infections

Cefazolin and ampicillin-sulbactam are the first-choice antimicrobials used in our country for methicillin-sensitive Staphylococcus aureus (MSSA) bacteremia. In recent years, it has been reported that there may be a decrease in the sensitivity of cefazolin in higher bacterial inoculum against some MSSA strains with some β-lactamase species, Studies concerning the possibility of treatment failure with cefazolin in patients infected with these strains have been published. There is no data on this subject in our country. All patients who received inpatient treatment at Istanbul University Istanbul Faculty of Medicine services between 2016-2018, who had a single blood culture positive for MSSA and who were evaluated as MSSA bacteremia or endocarditis and whose causative strain was kept were included in the study. The susceptibility of cefazolin, ampicillin-sulbactam, cefuroxime, cefotaxime and ceftriaxone were determined by disk diffusion and broth microdilution method (as MIC) in 105 standard inoculum and 107 high inoculum.β-lactamase presence in the strains were determined by using nitrocefine disc, penicillin disk diffusion method, penicillin zone edge test and detecting the blaZ gene by PCR method, the beta-lactamase type was defined by PCR and sequencing method. For this purpose, PCR was performed using primers F, 5'-CAA AGA TGA TAT AGT TGC TTA TTC-3 'and R, 5'-CAT ATG TTA TTG CTT GCA CCAC-3 ' which were designed to amplify the 355-bp region containing 128 and 216 residues in the blaZ gene structure. PCR products were then analyzed by DNA sequencing. S. aureus ATCC 29213 strain, which is known to produce a small amount of type A beta-lactamase, and blaZ negative S. aureus ATCC 25923 strain were used as control strains. Beta-lactamase gene was found in 84.6% of the strains (44/52). As a result of the sequence analysis of 40 strains, 100% of them were defined as type A beta-lactamase. Penicillin zone was significantly smaller in both inoculum in the presence of beta-lactamase (p=0.001). Cefazolin zone was significantly smaller in both inoculum in the presence of betalactamase (p=0.001). The MIC of cefazolin measured by microdilution method was significantly higher in the presence of betalactamase in the high and standard inoculum (p=0.028 and p=0.001). Cefazolin InE was seen 25%, 5.8% of them were resistant. It was found that the high inoculum MIC of cefazolin is an independent risk factor affecting mortality (p=0.049). The MIC value of ampicillin sulbactam measured by Etest® was significantly higher in the presence of betalactamase in standard and high inoculum (p=0.001). Ampicillin-sulbactam inoculum effect was seen in 36.5% of patients. In cefotaxime and cefuroxime inoculum effect of cefotaxime and cefuroxime was not observed. Mortality rate was found 15.4%. In univariate analysis, Charlson comorbidity index was found statistically significantly higher in patients who died (p = 0.031). In 8 patients who died, 4 had cefazolin inoculum effect (p=0.17), 5 had ampicillin-sulbactam inoculum effect (p=0.049) and a significant relationship was found between ampicillin-sulbactam InE and mortality in univariate analysis. In multivariate analysis, albumin value (OR = 0.013, 95% CI <0.001-0.789, p=0.038) and cefazolin MIC value in high inoculum (OR 1.8, 95% CI 1.002-3.261) were defined as independent risk factors affecting mortality. The sensitivity of nitrocefine test and penicillin disk diffusion method (penicillin zone diameter and zone edge test) in detecting β-lactamase was 100%.It was found that the MIC values of cefazolin and ampicillin-sulbactam increased significantly at high microorganism concentrations by 25% and 36.5% respectively, and the MIC value in high inoculum is an independent risk factor affecting mortality. Therefore, in high inoculum infections such as MSSA bacteremia or endocarditis, the inoculum effect of the causative strain should be considered when choosing the appropriate antimicrobial.