Indian Journal of Medical Microbiology Medknow Publications on behalf of Indian Association of Medical Microbiology ISSN: 0255-0857 EISSN: 1998-3646 Vol. 25, Num. 3, 2007, pp. 203-208

Indian Journal of Medical Microbiology, Vol. 25, No. 3, July-September, 2007, pp. 203-208

Original Article

Sensitivity pattern of gram negative bacilli to three β-lactam/β-lactamase inhibitor combinations using the automated API system

Anuradha K, Sailaja VV, Umabala P, Satheesh T, Lakshmi V

Department of Microbiology, Nizam's Institute of Medical Sciences, Hyderabad - 500 082, Andhra Pradesh
Correspondence Address: Department of Microbiology, Nizam's Institute of Medical Sciences, Hyderabad - 500 082, Andhra Pradesh, lgorthi@hotmail.com

Date of Submission: 29-Sep-2004
Date of Acceptance: 12-Dec-2005

Code Number: mb07060

Abstract

Keywords: API, β-lactamase inhibitors, gram negative bacteria, sensitivity

An extensive use of β -lactam antibiotics in hospitals and community has created major resistance problems leading to increased morbidity, mortality and health-care costs. ^[1] Of the several mechanisms of resistance, the most widespread and most important is the destruction of the β -lactam ring, which is mediated by β -lactamases. ^[2] Extended spectrum β -lactamases (ESBLs) are of greater concern because they are capable of hydrolyzing several groups of β -lactam antibiotics, notably third and fourth generation cephalosporins and extended spectrum penicillins such as piperacillin. There are 255 known β -lactamases to date and the continued use of β -lactams may select for newer variants. ^[3]

The use of β -lactamase inhibitors in combination with β-lactam antibiotics is currently the most successful strategy to combat this specific resistance mechanism. These β -lactamase inhibitors are thought to be "suicide inhibitors" that form stable complexes between the bacterial β -lactamase and the β -lactamase inhibitor in a multi-step chemical reaction. ^[4] Their broad spectrum of activity originates from the ability of respective inhibitors to inactivate a wide range of β -lactamases produced by gram-positive, gram-negative, anaerobic and even acid-fast pathogens. β -lactam/ β -lactamase inhibitor combinations are particularly useful against mixed infections and play a useful role in treating various multi resistant pathogens such as ESBLs. These agents are gaining importance in the treatment of various multi-resistant and emerging nosocomial pathogens such as Acinetobacter spp. and Stenotrophomonas maltophilia. ^[5]

This study compared the spectrum of activity of β-lactam/ β-lactamase inhibitors to that of cephalosporins against gram-negative bacilli.

Materials and Methods

The in vitro activity of three β -lactamase inhibitors in comparison with cephalosporins was evaluated against clinical isolates from patients admitted in our institute using the API system. Amoxycillin/clavulanic acid (AUG), ticarcillin/clavulanic acid (TIM) and piperacillin/tazobactam (TZP) and all the four generations of cephalosporins were included in the study.

A total of 1252 Enterobacteriaceae isolates and 385 isolates from non-fermenters were tested for AUG, TIM and TZP and TIM and TZP respectively. [Table - 1], [Table - 2] show all the isolates tested and the antibiotics used.

All the isolates were identified and their minimum inhibitory concentrations (MICs) were determined by API system (bioMιrieux) using the relevant panels. The inoculum was prepared as per the standard protocol given in the instruction manual of the API system. Quality control was performed by using Escherichia coli strain ATCC 25922 for Enterobacteriaceae and Pseudomonas aeruginosa strain ATCC 27853 for non-fermenters, with each new batch of strips.

Results

Among the Enterobacteriaceae , the overall resistance to amoxicillin/clavulanic acid, ticarcillin/ clavulanic acid and piperacillin/tazobactam was 82.92, 58.22 and 22.44% respectively [Table - 3] and 82-65% resistance to cephalosporins from first to fourth generations.

[Table - 4] shows the MICs of different β -lactam/β -beta-lactamase organisms to Enterobacteriaceae group of inhibitors and [Table - 5], [Table - 6], [Table - 7], [Table - 8], [Table - 9], [Table - 10] show the MICs against cephalosporins. [Table - 11] shows the MICs of different β -lactam/β -beta-lactamase inhibitors and cephalosporins to various non-fermenters.

Discussion

Penicillin, the first of the β -lactam antibiotics, was first introduced into medical practice in the 1940s. Since then, a large number of different β -lactams, including penicillins, cephalosporins, monobactams and carbapenems have been developed, all of which are structurally related through the presence of a core β -lactam ring. ^[2] The most common mechanism of resistance to beta-lactam antibiotics is the production of β -lactamase, which destroys β -lactam antibiotics before they reach the bacterial target. ^[6] A highly effective and proven approach for tackling β -lactamase mediated resistance to β -lactams is the use of the β -lactam/β -lactamase inhibitor combinations. ^[7] In recent years, the use of these combinations has been proven to be a useful and an effective strategy to improve upon the therapeutic value of β -lactam antibiotics. ^[8] Several factors influence the activity and pharmacodynamics of these combinations, including potency of these agents, pharmacokinetics of the inhibitor, type and quality of b-lactamase produced by the target bacterium and potential for the inhibitor to induce expression of chromosomal cephalosporinases in the target bacterium. ^[9] The overall antibacterial spectrum of these drug combinations depends on the intrinsic activity of the β -lactam as well as the characteristics of the individual inhibitor towards different β -lactamases. ^[5] The differences among these b-lactam/ β -lactamase inhibitors such as spectrum of activity, need to be considered in choosing an agent for a specific case.

Currently, there are three commercially available β -lactamase inhibitors-clavulanic acid, sulbactam and tazobactam. ^[7] These inhibitors are available in the combinations of amoxycillin/clavulanic acid (augmentin), ampicillin / sulbactum and piperacillin/tazobactam (tazact), ticarcillin/clavulanic acid (timentin) and cefoperazone/sulbactam (magnex). Tazobactam in combination with piperacillin has an excellent clinical efficacy in various infections, caused by class A, D and C β -lactamase producing bacteria, including ESBL producers. ^[10] The available API panels allowed MIC determinations of amoxycillin/clavulanic acid, piperacillin/tazobactam and ticarcillin/clavulanic acid.

In the present study, 71.06% of the isolates tested. were susceptible to piperacillin/tazobactam, 35.06% to ticarcillin/clavulanic acid and 16.38% to amoxicillin/clavulanic acid among Enterobacteriaceae . In another similar study, 91.7% of Enterobacteriaceae isolates were susceptible to piperacillin/tazobactam while ticarcillin/clavulanic acid was active against 85.8% isolates. ^[11] Another study showed susceptibility of> 74% against piperacillin/tazobactam,> 69% against ticarcillin/clavulanic acid and> 34% susceptibility against ampicillin/sulbactam. ^[12] As observed by other studies, ^[11],[12] piperacillin/tazobactam was documented as the most active β -lactam/β -lactamase inhibitor combination against Enterobacteriaceae in our study.

In the context of the non-fermenter isolates, piperacillin/tazobactam was found to be the most active combination against P. aeruginosa and ticarcillin/clavulanic acid against A. baumannii and S. maltophilia similar to the above studies ^{[11],[12],[13],[14]} [Table - 12].

Although all these study results indicated the same rank orders of activity, difference in the susceptibility rates was observed. The reasons could possibly be contributed to the hospital organisms sampled, test methods, sites of infection and the study time interval. ^[12] We found lower susceptibility rates to all the three β -lactam/β -lactamases compared to other studies. Referral and a tertiary care hospital status of our hospital and prior treatment with multiple antibiotics may account for high resistance among the isolates from this hospital.

In the treatment of P. aeruginosa infections, the potential for clavulanic acid to induce expression of chromosomal cephalosporinase and antagonize antibacterial activity of ticarcillin is a concern, especially in patients who lack protective host defences. These are not concerns with piperacillin/tazobactam. ^[9] Tazobactam seems to be the most promising β -lactamase inhibitor, which has, unlike clavulanic acid and sulbactam, its own antibiotic activity. ^[15]

Multiple antibiotic resistance is becoming increasingly prevalent in the opportunistic pathogen A. baumannii . Ticarcillin/clavulanic acid was found to be the most effective compared to piperacillin/tazobactam against A. baumannii .

Though sulbactam has the highest intrinsic activity, compared to the other inhibitors, against A. baumannii ^{[9],[16],[12]} ampicillin/sulbactam was not tested in our study as it was not included in the susceptibility strip used for testing non-fermenters of the API.

The majority of clinical isolates of the recently emerging nosocomial pathogen S. maltophilia , are resistant to multiple antibiotics. Ticarcillin/clavulanic acid combination is one of the few agents that have greater activity against this pathogen than the other β -lactam/β -lactamase inhibitor combinations. ^[7] The same pattern was observed in the present study.

The high resistance exhibited by all isolates included in this study to all generation of cephalosporins as compared to β -lactam/β -lactamase inhibitors may be due to an increased use of cephalosporins in our hospital. Under a selective pressure induced by the extensive use of the cephalosporins, especially the third generation, ESBL producers appear and spread within the hospital.

β -lactam/β -lactamase inhibitors can be useful alternatives to conventional two-three drug regimens in mixed infections, such as foot infections in patients with diabetes mellitus and hospital-acquired intra-abdominal infections. ^[17] Their substitution in place of cephalosporins appears to reduce emergence of the ESBL producing pathogens. Similarly their use may also curtail the emergence of other resistant pathogens such as Clostridium difficile and vancomycin resistant Enterococci . These are generally well-tolerated and their oral forms provide effective outpatient therapy against many commonly encountered infections. ^[5] They could even be more cost-effective than conventional combination therapies. ^[5]

Piperacillin/tazobactam was found to be the most active combination of the three against Enterobacteriaceae and Pseudomonas spp. and ticarcillin/clavulanic acid against Acinetobacter spp. and Stenotrophomonas maltophilia . The isolates exhibited high resistance to other groups of antibiotics except carbapenems.

References

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