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. 2017 Jan-Mar;48(1):159-166.
doi: 10.1016/j.bjm.2016.10.011. Epub 2016 Nov 11.

Correlation of phenotypic tests with the presence of the blaZ gene for detection of beta-lactamase

Adriano Martison Ferreira  1 , Katheryne Benini Martins  2 , Vanessa Rocha da Silva  2 , Alessandro Lia Mondelli  3 , Maria de Lourdes Ribeiro de Souza da Cunha  4
Affiliations

Correlation of phenotypic tests with the presence of the blaZ gene for detection of beta-lactamase

Adriano Martison Ferreira et al. Braz J Microbiol. 2017 Jan-Mar.

Abstract

Staphylococcus aureus and Staphylococcus saprophyticus are the most common and most important staphylococcal species associated with urinary tract infections. The objective of the present study was to compare and to evaluate the accuracy of four phenotypic methods for the detection of beta-lactamase production in Staphylococcus spp. Seventy-three strains produced a halo with a diameter ≤28mm (penicillin resistant) and all of them were positive for the blaZ gene. Among the 28 susceptible strain (halo ≥29mm), 23 carried the blaZ gene and five did not. The zone edge test was the most sensitive (90.3%), followed by MIC determination (85.5%), but the specificity of the former was low (40.0%). The nitrocefin test was the least sensitive (28.9%). However, the nitrocefin test together with the disk diffusion method showed the highest specificity (100%). The present results demonstrated that the zone edge test was the most sensitive phenotypic test for detection of beta-lactamase, although it is still not an ideal test to detect this type of resistance since its specificity was low. However, the inhibition halo diameter of the penicillin disk can be used together with the zone edge test since the same disk is employed in the two tests. Combined analysis of the two tests shows a sensitivity of 90.3% and specificity of 100%, proving better sensitivity, especially for S. saprophyticus. This is a low-cost test of easy application and interpretation that can be used in small and medium-sized laboratories where susceptibility testing is usually performed by the disk diffusion method.

Keywords: Beta-lactamase; Staphylococcus saprophyticus; Urinary tract infection; Zone edge test; blaZ.

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Figures

Fig. 1
Fig. 1
Resistance to penicillin by the disk diffusion method and E-test®.
Fig. 2
Fig. 2
Positive zone edge test for the detection of beta-lactamase production (sharp edge).
Fig. 3
Fig. 3
Negative zone edge test for the detection of beta-lactamase production (fuzzy edge).
Fig. 4
Fig. 4
Agarose gel electrophoresis for detection of the blaZ gene (421 bp) by PCR. Lanes 4–8 and 13–22: positive isolates; line 9 is positive an 13 is negative: negative isolates; 3: negative control (S. aureus ATCC 25923); 2: positive control (S. aureus ATCC 29213); 23: water; 1: molecular weight marker (100 bp).
Fig. 5
Fig. 5
Correlation between halo size and presence of the blaZ gene.
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