Utility of anaerobic bottles for the diagnosis of bloodstream infections

BACKGROUND

Obligate anaerobes usually account for less than 10% of bacteria recovered from blood cultures (BC). The relevance of routine use of the anaerobic bottle is under debate. The aim of this study was to evaluate the utility of anaerobic bottles for the diagnosis of bloodstream infections (BSI).

METHODS

We conducted a 6-month, retrospective, monocentric study in a tertiary hospital. All positive BC were grouped into a single episode of bacteremia when drawn within 7 consecutive days. Bacteremia were classified into contaminants and BSI. Charts of patients with BSI due to obligate anaerobes were studied.

RESULTS

A total of 19,739 blood cultures were collected, 2341 of which (11.9%) were positive. Anaerobic bottles were positive in 1528 (65.3%) of all positive BC but were positive alone (aerobic bottles negative) in 369 (15.8%). Overall 1081 episodes of bacteremia were identified, of which 209 (19.3%) had positive anaerobic bottles alone. The majority 126/209 (60.3%) were contaminants and 83 (39.7%) were BSI. BSI due to facultative anaerobes, obligate aerobes and obligate anaerobes were identified in 67 (80.7%), 3 (3.6%) and 13 (15.7%) of these 83 episodes, respectively. BSI due to obligate anaerobic bacteria were reported in 9 patients with gastro-intestinal disease, in 3 with febrile neutropenia and in 1 burned patient.

CONCLUSIONS

Anaerobic bottles contributed to the diagnosis of a significant number of episodes of bacteremia. Isolated bacteria were mostly contaminants and non-obligate anaerobic pathogens. Rare BSI due to obligate anaerobes were reported mainly in patients with gastro-intestinal disorders and during febrile neutropenia.

In vivo selection of a complex mutant TEM (CMT) from an inhibitor-resistant TEM (IRT) during ceftazidime therapy

OBJECTIVES

A relapse from Escherichia coli bloodstream infection was observed in a patient with acute leukaemia treated with ceftazidime for 7 days for febrile neutropenia. Whereas the original E. coli isolate was resistant to β-lactam/β-lactamase inhibitor combinations (EC1), the relapse E. coli isolate showed a similar phenotype but with resistance extended to ceftazidime (EC2). We investigated the molecular mechanisms of β-lactam resistance and sought if EC2 could have been selected in vivo from EC1.

METHODS

EC1 and EC2 isolates were compared for antibiotic MICs, plasmid content, genotyping, β-lactamase genes and their environment. Both isolates were conjugated with E. coli JW4111ΔampC and MICs determined for transconjugants. In addition, ceftazidime-resistant mutants were selected in vitro from EC1.

RESULTS

EC1 and EC2 showed identical patterns for genotyping and resistance plasmids. PCR sequencing of blaTEM in EC1 showed the mutations M69L and N276D corresponding to TEM-35, also called inhibitor-resistant TEM (IRT)-4. In EC2, the TEM allele showed an additional mutation, R164S, known to confer resistance to ceftazidime. The combination of these three mutations was previously reported in TEM-158, described as the complex mutant TEM (CMT)-9, associated with resistance to β-lactamase inhibitors and third-generation cephalosporins. In vitro selection of ceftazidime-resistant mutants from EC1 yielded six different CMT alleles, including TEM-158 containing the R164S mutation.

CONCLUSIONS

This first known report of in vivo selection of CMT from IRT, reproduced in vitro, shows how the evolution of β-lactamase enzymes is easily driven by antibiotic pressure, even during a short antibiotic therapy.