Antimicrobial susceptibility of respiratory <i>Haemophilus influenzae</i> strains isolated from pediatric respiratory tract infections

A higher inoculum size of beta-lactamase-positive Haemophilus influenzae is reported to increase minimum inhibitory concentrations (MICs) for beta-lactams. However, the effect of inoculum size of beta-lactamase-negative, ampicillin-resistant H. influenzae (BLNAR) on MICs for carbapenems has not been investigated. This study evaluated the effect of inoculum size on MICs for carbapenems and other beta-lactams in nine clinical isolates of BLNAR. The MICs were determined by both the standard method described by the Clinical and Laboratory Standards Institute (final inoculum size of 5 x 10(5) colony-forming units [CFU]/ml) and a modified method (final inoculum size of 5 x 10(6) CFU/ml) using viable cell counts. The findings showed that the higher inoculum size increased MICs for imipenem, meropenem, panipenem, biapenem, ampicillin, ceftazidime, and ceftriaxone. The inoculum effect (4 log(2) dilution or a greater increase in the MIC) with imipenem, meropenem, panipenem, and biapenem was found in three, five, two, and two isolates, respectively. The magnitude of the inoculum effect for panipenem significantly increased with the levels of MICs, but correlation between them for the others was not statistically significant. The mutations of penicillin-binding protein genes had little relevance to the reduced susceptibility to carbapenems or to the magnitude of the inoculum effect. These results suggest that MIC determination using turbidity can produce interpretive errors in the antimicrobial susceptibility testing of BLNAR for carbapenems because of their inoculum effect. Thus, accurate adjustment of inoculum size, such as viable cell count, is helpful for confirming the true MICs when the isolates are interpreted as "resistant" by turbidity-based MIC determination.

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Effect of the Inoculum Size on Carbapenem Susceptibilities of β-Lactamase-Negative, Ampicillin-Resistant Haemophilus influenzae

Miyazaki

Horii

Nagura

et al. 2008

Curr Microbiol

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Incidence of bacterial coinfection with respiratory syncytial virus bronchopulmonary infection in pediatric inpatients

Hishiki

Ishiwada

Fukasawa

et al. 2011

Journal of Infection and Chemotherapy

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Serotype distribution and β-lactam resistance in Haemophilus influenzae isolated from patients with respiratory infections in Korea

et al. 2010

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Haemophilus influenzae is a frequent causative bacterial pathogen of respiratory tract infections. Resistance to beta-lactam antibiotics has been a significant clinical problem in treatment for H. influenzae respiratory infections. This study describes the serotype, antibiotic resistance and distribution of TEM-1 or ROB-1 beta-lactamase in H. influenzae isolates from local private hospitals from 2002 to 2004. Among the 100 H. influenzae respiratory isolates, only 7% were identified as serotypes a, b, e, and f, with the remaining 93% being nontypeable. Resistance to ampicillin, cefaclor, and tetracycline was 57%, 46%, and 16%, respectively. All strains were susceptible to azithromycin and ciprofloxacin, whereas amoxicillin/clavulanate, cefotaxime, and imipenem exhibited reduced susceptibilities of 99%, 99%, and 91%, respectively. All 57 ampicillin-resistant strains (minimum inhibitory concentration, MIC>or=4 microg/ml) were beta-lactamase-positive and possessed the TEM-1 type beta-lactamase. One beta-lactamase-positive amoxicillin/clavulanate-resistant isolate that was resistant to ampicillin (MIC>128 microg/ml) had the TEM-1 type beta-lactamase and not susceptible to cefaclor and cefotaxime. Analysis of penicillin binding protein 3 revealed six residues (Asp-350, Met-377, Ala-502, Asn-526, Val-547, and Asn-569) that were substituted by Asn, Ile, Val, Lys, Ile, and Ser, respectively.

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Antimicrobial susceptibility of respiratory Haemophilus influenzae strains isolated from pediatric respiratory tract infections

Abstract: Frequency of BLNAR strains causing respiratory infections has been dramatically increasing, which can diminish antibiotic activities of not only ampicillin but also of some cephems.

Cited by 6 publications

References 17 publications

Effect of the Inoculum Size on Carbapenem Susceptibilities of β-Lactamase-Negative, Ampicillin-Resistant Haemophilus influenzae

Effect of the Inoculum Size on Carbapenem Susceptibilities of β-Lactamase-Negative, Ampicillin-Resistant Haemophilus influenzae

Incidence of bacterial coinfection with respiratory syncytial virus bronchopulmonary infection in pediatric inpatients

Serotype distribution and β-lactam resistance in Haemophilus influenzae isolated from patients with respiratory infections in Korea

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