Introduction

Multiple commercial systems for rapid identification (ID) and antimicrobial susceptibility testing (AST) of bacteria directly from positive blood cultures (BC) have been developed recently. However, these systems are associated with considerable costs. In this study, we aimed to use the lytic property of the BD BACTEC™ Lytic/10 Anaerobic/F BC bottles for the simplified and cost-saving isolation of bacterial cells for downstream ID and AST by standard methods.

Methods

During seven months, all anaerobic BCs with Gram-negative rods were included in the study. 1.5 ml of blood were centrifuged for 1 min at 13.000 rpm and washed once with NaCl. The cell pellet obtained was used directly for ID using MALDI-TOF, AST using VITEK® 2 (bioMérieux) and detection of third generation cephalosporin resistance using the colorimetric β-LACTA™ test (Bio-Rad). The results were compared with the standard-of-care (MALDI-TOF and VITEK® 2 from short-term culture).

Results

104 positive anaerobic BCs, accounting for 80.7% of all BCs with Gram-negative rods, were processed. 16 (15.4%) of them were excluded because of a mixed culture (n=11) or the detection of anaerobic bacteria (n=5), leaving 88 BCs for analysis. In total, 14 different bacterial species were analyzed, mostly Escherichia coli (58%). ID with MALDI-TOF was correct at the species level in 93% of cases (mean score 2.18; standard deviation 0.19). Within 30 min from BC-positivity, the β-LACTA™ test identified 7 out of 8 isolates with resistance to 3rd generation cephalosporins. Complete VITEK® 2 AST results were available in the majority of cases until 8 p.m.. Most antibiotics showed an essential and categorical agreement above 96%. Few very major errors occurred with ampicillin/sulbactam (n=2) and moxifloxacin (n=2).

Conclusions

The lytic BD BACTEC™ anaerobic BC bottles enable a fast and simple workflow to accelerate ID and AST from positive BCs at minimal costs and hands-on-time.