Journal of Postgraduate Medicine
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Year : 1989  |  Volume : 35  |  Issue : 3  |  Page : 147-51  

Correlation between beta-lactamase production and MIC values against penicillin with coagulase negative staphylococci.

TV Narayani, JJ Shanmugam, KK Naseema, RN Bhattacharya, KG Shyamkrishnan 

Correspondence Address:
T V Narayani


Two hundred strains of coagulase negative staphylococci (CNS) isolated from various clinical specimens (116) and healthy hospital personnel (84) were investigated for the production of beta-lactamases by means of three iodometric techniques and correlated with the minimum inhibitory concentration (MIC) values of penicillin-G by agar dilution technique and disc diffusion technique. One hundred and fifty (75.0%) of the 200 strains tested produced beta-lactamases. Seventy two per cent of the CNS were found to be beta-lactamase positive by the starch paper technique which was the most sensitive one in our study. The MIC values of penicillin against CNS ranged from less than or equal to 1.25 to greater than or equal to 2000 units. The present study indicated the higher prevalence of beta-lactamase producers with increased penicillin resistance among CNS strains isolated from healthy carriers and hospitalised patients.

How to cite this article:
Narayani T V, Shanmugam J J, Naseema K K, Bhattacharya R N, Shyamkrishnan K G. Correlation between beta-lactamase production and MIC values against penicillin with coagulase negative staphylococci. J Postgrad Med 1989;35:147-51

How to cite this URL:
Narayani T V, Shanmugam J J, Naseema K K, Bhattacharya R N, Shyamkrishnan K G. Correlation between beta-lactamase production and MIC values against penicillin with coagulase negative staphylococci. J Postgrad Med [serial online] 1989 [cited 2023 Mar 23 ];35:147-51
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Historically, coagulase negative staphylococci (CNS) have been of lesser medical concern than Staphylococcus aureus. Recently, these organisms have attracted more attention as human pathogens.[4],[12] Infections caused by CNS have occurred in association with implantable medical devices such as ventriculo-peritoneal shunts, prosthetic heart valves, vascular grafts and hip prostheses.[8] Infections by CNS can prove difficult to treat either because the CNS are associated with prostheses or because they have an unpredictable sensitivity pattern.[5] Clinically significant resistance of CNS strains to penicillin is correlated with the production of penicillinase or beta-lactamases. Different methods are used for the detection of beta-lactamases namely acidometric, iodometric, chromogenic cephalosporin and microbiological methods.[15]

The iodometric assay for penicillinase production is based on the fact that the intact (active) penicillin molecule does not bind iodine whereas the beta-Lactamase inactivated product penicilloic acid binds iodine. Thus a positive reaction indicates that iodine, being bound to penicilloic acid is unavailable for further reaction with starch and so no purple colour develops in testing.[9]


A total of 200 strains of CNS from healthy carriers (84 strains) and clinical specimens namely blood, urine and pus (116 strains) were tested for the presence of betalactamases by three different iodometric techniques viz. tube test, starch agar plate and starch filter paper methods. The first two tests were carried out as described by Sykes[14] and the third one as per the method of Odugbemi et al.[11] Besides we also carried out disc diffusion[2] and agar dilution susceptibility test' to correlate penicillin-G minimal inhibitory concentration with beta-lactamase production and disc susceptibility. The agar dilution method was carried out by preparing two fold dilutions of the antibiotics in sterile distilled water starting from 1.25 units to 2000 units and mixed with melted and cooled Muller-Hinton agar (MHA). Each dilution was poured into labelled petri dishes and allowed to solidify. Two to three hour old nutrient broth culture was inoculated by means of a 2 mm diameter inoculation loop on each plate.


Out of a total of 200 strains tested for beta-lactamase production, 150 were positive (75.0%). There was no significant difference of beta-lactamase production between the carrier (64/84 = 76.2%) and clinical (84/116 = 74.1 %) isolates. [Table 1]shows the results obtained by three different methods of beta-lactamase detection. Starch paper technique detected maximum beta-lactamase producers in both carrier and clinical strains (73.8% and 69.8% respectively) followed by agar plate and test tube techniques. Weak beta-lactamase producers were detected mainly by the starch paper technique (51.6%-carrier strains and 55.6%-clinical strains) among betalactamase producers. Only 17.5% of betalactamase producers could be detected by one of the three methods used whereas 75.0% positivity was obtained by using all three methods. This shows that a combination of methods is always useful in detecting beta-lactamase producers [Table 2]. The correlation between beta-lactamase production and penicillin susceptibility pattern is shown in[Table 3]. Totally, 18.4% of the strains were sensitive to penicillin by disc diffusion but were positive for betalactamases indicating insufficient amount of the enzymes to hydrolyse penicillin. Such weak beta-lactamase producers were detected mainly by the starch paper technique. Out of all, 7.9% of the strains were resistant to penicillin by disc diffusion and at the same time negative for beta-lactamase production indicating that the resistence is not due to beta-lactamases.

The MIC values of the CNS ranged from = 1. 25 units to = 2000 units. The highest dilution of penicillin-G in MHA in which no visible growth was seen after overnight incubation at 37 C was taken as the MIC of the test strain. Among the penicillin resistant carrier strains, 53.5% showed an MIC value ranging from = 1.25 to 50 units while among the clinical strains only 38.4% showed similar MIC values. Higher MIC values (100 to = 2000 units) of penicillin resistant strains was 46.5% with carrier strains. In contrast, 61.6% of clinical strains showed similar higher values of MIC [Table 4].


For many years penicillin has been the most trusted and useful antibiotic in clinical practice. However, as a result of its wide spread and prolonged usage, many pathogenic bacteria have become resistant to it. In many cases, penicillin resistance results from the bacterium having acquired the ability to produce penicillinase.[3] Gill et al[7] found 78.6% of the clinical CNS strains positive for beta-lactamase detection by acidometric techniques.[7] Our finding of 74.1% of positivity with clinical CNS strains is comparable with the above report. Rosenblatt et al[13] have tested fifteen clinical strains of S.epidermidis for beta-lactamase by the slide test and all were found to be negative for beta-lactamase production. The above strains showed an MIC value of = 0. 5 mcg/ml. In our study the beta-lactamase negative clinical strains showed an MIC range between = 1.25 to 50 units except one strain having 100 units. Oberhofer and Towle[10] compared the penicillin susceptibility pattern by disc diffusion and beta-lactamase detection by starch paper strip method with clinical strains of S.epidermidis. They have reported cent per cent correlation between beta-lactamase production and penicillin susceptibility pattern. In our study 85.5% of the clinical strains were resistant to penicillin and correlated with beta-lactamase production as tested by the starch paper strip method. In contrast, 22.6% were positive for betalactamase production even though they were sensitive to penicillin by disc diffusion technique. Gemmell and Dawson[6] analysed the strains according to the sources of isolates. The isolates from blood, urinary tract and miscellaneous skin infections differ little with respect to antibiotic susceptibility pattern-the rate of resistance being 30%, 37.7% and 40.7% respectively.[6] In our study, the urinary tract isolates showed an increased percentage of penicillin resistance (74.3%) followed by strains from wound infections (52.2%) and blood isolates (47.4%). Determination of MIC values together with the detection of beta-lactamase production and antibiotic susceptibility pattern will yield valuable information for initiating effective antibiotic therapy for an underlying infection as well as in understanding and controlling hospital acquired infections due to CNS species, which are recently emerging as pathogens.


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