Ampicillin disk diffusion susceptibility testing of Haemophilus influenzae

Doern, Gary V.; Daum, Gary S.; Tubert, Tracy A.

doi:10.1128/jcm.25.9.1675-1678.1987

Cited by 12 publications

(7 citation statements)

References 11 publications

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“…Possible alternative mechanisms of AMP resistance include altered penicillin-binding proteins and diminished permeation of beta-lactam antimicrobial agents through the cell wall outer membrane (5,12). Isolates with this mechanism are considered rare (6,18).…”

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confidence: 99%

Investigation of ampicillin-intermediate strains of Haemophilus influenzae by using the disk diffusion procedure and current National Committee for Clinical Laboratory Standards guidelines

1992

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It was noted in our laboratory that certain strains of Haemophilus influenzae yielded zone sizes interpreted as resistant to the ampicillin (AMP) disk on chocolate-Mueller-Hinton agar (CMH) but showed no evidence of 13-lactamase (13-Lac) activity. Although it is known that a second mechanism of AMP resistance exists, strains with this mechanism are uncommon. To investigate this apparent discrepancy, a study of 100 consecutive clinical isolates of H. influenzae collected over a 6-month period was performed. Isolates were simultaneously tested against five antibiotics (AMP, chloramphenicol, cefotaxime, ciprofloxacin, and AMP-sulbactam) on CMH and on two brands of Haemophilus test medium (HTM) by using the disk diffusion procedure and National Committee for Clinical Laboratory Standards (NCCLS) standards. By using CMH and NCCLS standard M2-A3-S2, strains of H. influenzae showing zone sizes of .20 mm with AMP were considered sensitive. By using HTM and NCCLS standard M2-A4, strains showing zone sizes of .25 mm to AMP on HTM were considered sensitive. Intermediate strains had zone sizes of 22 to 24 mm. The majority of isolates (68%) were sensitive to all antibiotics. Two percent of the isolates were resistant to chloramphenicol. Seventeen percent of the isolates were AMP-resistant, 13-Lac-producing strains of H. influenzae. Thirteen percent of the isolates gave at least one intermediate or resistant zone for AMP but were 13-Lac negative. MIC determinations with NCCLS standard M7-A2 were performed with resistant and intermediate strains. MICs for 13-Lacproducing strains of H. influenzae were .8.0 ,ug/ml. MICs for 13-Lac-negative strains were '1.0 ,ug/ml and were highly reproducible. If one uses the current NCCLS zone diameter interpretive criteria, results should be viewed with caution. Further investigation of zone size interpretive criteria is warranted. It is suggested that in the case of serious infections with H. influenzae, 13-Lac-negative, AMP-resistant or -intermediate strains be confirmed by the MIC procedure.

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confidence: 99%

Investigation of ampicillin-intermediate strains of Haemophilus influenzae by using the disk diffusion procedure and current National Committee for Clinical Laboratory Standards guidelines

1992

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“…Clearly, medium composition is crucial to susceptibility test results with Haemophilus spp. (1,(4)(5)(6)17). Perhaps the most widely used medium for disk testing of H. influenzae, at least in the United States, has been chocolatized MH agar (MHchoc) (25).…”

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In vitro susceptibility testing of Haemophilus influenzae: review of new National Committee for Clinical Laboratory Standards recommendations

Doern

1992

J Clin Microbiol

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“…Mendelman et al (19) comparing two ampicillin disks and using a small number of selected H. influenzae isolates, reported that the 10-g disk test failed to detect 44% of BLNAR strains. Another study evaluating a larger number of clinical isolates and two different media (7), found that all medium-ampicillin disk result combinations were generally equivalent in detecting ampicillinsusceptible and -resistant strains. The best accuracy, however, was demonstrated by the 2-g ampicillin disk tested on chocolate agar.…”

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“…This includes the optimization and standardization of susceptibility tests, the establishment of interpretive criteria for MIC and disk diffusion methods, as well as the development of guidelines for quality control (QC) procedures. For the susceptibility testing of H. influenzae a special medium, the Haemophilus Test Medium (HTM), and an appropriate standardized methodology (6,7,9,15) have been recommended by National Committee for Clinical Laboratory Standards (NCCLS) (21,22). All previously reported clinical isolates of H. influenzae appear susceptible to meropenem, with MICs ranging from 0.06 to 1 g/ml (13,25,33).…”

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Reevaluation of interpretive criteria for Haemophilus influenzae by using meropenem (10-microgram), imipenem (10-microgram), and ampicillin (2- and 10-microgram) disks

1996

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A collection of 300 Haemophilus influenzae clinical strains was used to assess in vitro susceptibility to carbapenems (meropenem, imipenem) by MIC and disk diffusion methods and to compare disk diffusion test results with two potencies of ampicillin disks (2 and 10 g). The isolates included ampicillin-susceptible or-intermediate (167 strains), ␤-lactamase-positive (117 strains), and ␤-lactamase-negative ampicillin-resistant (BLNAR; 16 strains) organisms. Disk diffusion testing was performed with 10-g meropenem disks from two manufacturers. Meropenem was highly active against H. influenzae strains (MIC 50 , 0.06 g/ml; MIC 90 , 0.25 g/ml; MIC 50 and MIC 90 , MICs at which 50 and 90%, respectively, of strains are inhibited) and was 8-to 16-fold more potent than imipenem (MIC 50 , 1 g/ml; MIC 90 , 2 g/ml). Five non-imipenem-susceptible strains were identified (MIC, 8 g/ml), but the disk diffusion test indicated susceptibility (zone diameters, 18 to 21 mm). MIC values of meropenem, doxycycline, ceftazidime, and ceftriaxone for BLNAR strains were two-to fourfold greater than those for other strains. The performance of both meropenem disks was comparable and considered acceptable. A single susceptible interpretive zone diameter of >17 mm (MIC, <4 g/ml) was proposed for meropenem. Testing with the 2-g ampicillin disk was preferred because of an excellent correlation between MIC values and zone diameters (r ‫؍‬ 0.94) and superior interpretive accuracy with the susceptible criteria at >17 mm (MIC, <1 g/ml) and the resistant criteria at <13 mm (MIC, >4 g/ml). Among the BLNAR strains tested, 81.3% were miscategorized as susceptible or intermediate when the 10-g ampicillin disk was used, while the 2-g disk produced only minor interpretive errors (12.5%). Use of these criteria for testing H. influenzae against meropenem and ampicillin should maximize reference test and standardized disk diffusion test performance with the Haemophilus Test Medium. The imipenem disk diffusion test appears compromised and should be used with caution for detecting strains for which imipenem MICs are elevated.

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Ampicillin disk diffusion susceptibility testing of Haemophilus influenzae

Cited by 12 publications

References 11 publications

Investigation of ampicillin-intermediate strains of Haemophilus influenzae by using the disk diffusion procedure and current National Committee for Clinical Laboratory Standards guidelines

Investigation of ampicillin-intermediate strains of Haemophilus influenzae by using the disk diffusion procedure and current National Committee for Clinical Laboratory Standards guidelines

In vitro susceptibility testing of Haemophilus influenzae: review of new National Committee for Clinical Laboratory Standards recommendations

Reevaluation of interpretive criteria for Haemophilus influenzae by using meropenem (10-microgram), imipenem (10-microgram), and ampicillin (2- and 10-microgram) disks

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