Introduction

Staphylococcus aureus infections are important causes of nosocomial and community acquired infections. Treatment of these infections is a growing problem due to increasing Methicillin resistance among Staphylococci.¹ The Macrolide-Lincosamide- Streptogramin B (MLS_B) family of antibiotics serve as an alternative treatment option, with clindamycin being the preferred agent due to its excellent pharmacokinetic properties.² However, widespread use of MLS_B antibiotics has led to an increase in number of Staphylococcal strains acquiring resistance to MLS_B antibiotics.³ Macrolide resistance may be due to enzymes encoded by a variety of erm genes- MLS_B phenotype or active efflux pump encoded by the mrsA gene- MS phenotype. MLS_B resistance can be either constitutive (c MLS_B) or inducible (i MLS_B). ⁴ In constitutive resistance, in vitro susceptibility tests show resistance to both erythromycin and clindamycin. In inducible resistance, in vitro susceptibility tests show resistance to erythromycin, but susceptibility to clindamycin, unless induced by erythromycin. iMLS_B resistance can not be determined by using standard susceptibility test methods, but can be determined by erythromycin- clindamycin disc approximation test (D test).

The aim of the present study was to determine the prevalence of inducible clindamycin resistance in both Methicillin resistant and susceptible strains of Staphylococcus aureus in clinical isolates and also susceptibility pattern of the isolates in our hospital.

Subjects and Methods

The present study was conducted for a period of 2 years from January 2007 to December 2008 and included a total of 140 isolates of Staphylococcus aureus from specimens of Pus/Wound swabs, Respiratory tract infections and Body fluids. The S. arueus were identified by using standard microbiological procedures.⁵ Antibiotic susceptibility tests were performed by Kirby Bauer disc diffusion method on Mueller Hinton agar plates using Erythromycin (15µg),Clindamycin (2µg),Penicillin(10U),Cefazolin (30µg) Ciprofloxacin(5µg) Oxacillin (1µg), Trimethoprim-Sulfomethaxazole (1.25/23.75µg ),Tetracycline (30µg)  Vancomycin ( 30µg) and Teicoplanin (30µg) as per Clinical Laboratory Standards Institute (CLSI) guidelines Methicillin resistance was detected by Oxacillin disc diffusion method.⁶

To identify i MLS_B phenotype, the D test was performed. A lawn culture of the isolate which was adjusted to 0.5 McFarland’s concentration was made on Mueller Hinton agar plate and discs of clindamycin (2µg) and erythromycin (15µg) were placed at a distance of 15mm (edge to edge) as per CLSI recommendations.

Four different phenotypes were interpreted as follows-

D positive (i MLS_B phenotype):-isolates showing resistance to erythromycin, while being sensitive to clindamycin with a D shaped zone of inhibition around clindamycin with flattening toward erythromycin disc. (Fig.1) D negative (MSB phenotype):-

No flattening of the clindamycin zone, resistant to erythromycin but susceptible to clindamycin.(Fig.3).

Constitutive resistance(c MLS_B phenotype):- Resistant to both erythromycin toclindamycin. (Fig. 2) Sensitive phenotype: – Sensitive to both erythromycin to clindamycin.

Figure 1:  D positive (i MLSB phenotype).   Click here to View figure

Figure 2:  Constitutive resistance(c MLSB phenotype).   Click here to View figure

Figure 3:  D negative (MSB phenotype).   Click here to View figure

Results

A total of 140 Staphylococci were isolated from various types of clinical specimens. Out of 140 isolates, 76 were Staphylococcus aureus and 64 were Methicillin resistant Staphylococcus aureus (MRSA).

The erythromycin and clindamycin resistance patterns of the isolates based on disc diffusion method are shown in the table- 1.

Table 1:- Distribution of MRSA and MSSA with different resistant phenotypes

cMLS_B phenotype (12.5%) was predominant in MRSA.  Sensitive phenotype was significantly higher in MSSA (84.21%). No cMLS_B phenotype was detected in MSSA.

iMLS_B phenotype (6.42%) was slightly higher than c MLS_B phenotype (5.71%) among the isolates.

The antibiotic susceptibility patterns of i MLS_B isolates are shown in the table.2. All isolates were susceptible to Vancomycin and Teicoplanin while all were resistant to penicillin and Ciprofloxacin. Sensitivity was least to Trimethoprim- Sulfomethaxazole and Tetracycline.

Table 2:- Sensitivity of   i MLS_B isolates to antimicrobial agents  . 

Discussion

Due to the increasing Nosocomial MRSA infections which are multidrug resistant treatment option is very limited. Vancomycin due to its high cost and possibility of emergence of resistance; it is not widely used by clinicians.⁷ Clindamycin due to its tolerability, cost, oral form and good tissue penetration remains alternative treatment option for skin and soft tissue infections. ⁸ But there are reports of iMLS_B resistance in the clinical isolates. ^{2,3,4,7,8,9,10,11,12} So it is essential to detect the inducible resistance to avoid therapeutic failure in patients.

In our study iMLS_B resistance was 6.42%. cMLS_B resistance was 5.71%. Similar findings of higher inducible resistance were reported by Angel et al (64% vs. 00%) and Shantala G B et al (24.89% vs. 18.26%).^9,10 Deotale et.al (14.5% vs. 3.6%). ¹¹ However in MRSA cMLS_B resistance (12.5%) was higher than that of iMLS_B resistance (10.93%).Many studies have reported similar results- Gadepalli et al reported 38% cMLSB resistance and 30 % iMLSB resistance. ³ Gupta et al reported 46% cMLSB resistance and 20 % iMLSB resistance. ¹² Debdas et al reported 23% cMLSB resistance and 18 % iMLSB resistance.¹³ Interestingly cMLSB resistance was not seen in MSSA. Similar result was reported by Angel et al only.

The antibiotic susceptibility patterns of i MLS_B isolates in this study showed that treatment options are very limited due to multidrug resistant strains. So clindamycin is the preferred drug for the treatment and for that D test must be performed to avoid therapeutic failure.

In conclusion, to avoid the therapeutic failure, the D test must be performed by all laboratories routinely while reporting the sensitivity against clindamycin in staphylococcal infections.

References

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