Susceptibility of Group A Streptococcus to Antimicrobial Agents in Northern Israel: A Surveillance Study

Introduction

Resistance to antimicrobials has been recognized as a major global public health threat for over a decade ¹⁵ and may pose a perpetual challenge. ⁷ As the scope of the problem escalates, considerable resources are being directed to combat it,^14,31 focusing on the appropriate use of antibiotics. Group A β-hemolytic streptococcus (Streptococcus pyogenes—GAS) causes a variety of infections, including life-threatening diseases; yet, the most common is acute tonsillitis. Although GAS is uniformly penicillin sensitive, resistance to other antimicrobials is often reported. ²⁰ Antimicrobial treatment for GAS is recommended, mainly in children and young people, due to the risk of developing acute rheumatic fever. Pharyngitis is a common complaint in primary care, particularly among children, being GAS responsible for 5–15% of the cases in adults and 15–35% in children.^2,5,12 Antimicrobial treatment is indicated following confirmation of GAS by culture or rapid antigen detection kits. ²⁶ Penicillin, the first choice of empiric treatment, has not developed bacterial resistance until now. Alternative antibiotics are required only for those patients reporting allergies or intolerance to penicillin ²⁶ ; hence, antimicrobial susceptibility of GAS is not routinely tested in our laboratory. A wide variability in antimicrobial susceptibility of GAS to macrolides and tetracyclines has been reported in several geographical regions.^{3,4,6,10,12,16,19,23,27–29,32} Recent published data from Europe indicate an increase of resistance to macrolides, which is quite a common practice in our region. A nationwide study conducted in Israel during 1996 to 1999 studied 500 GAS isolates from invasive infections. All strains were susceptible to penicillin, but 27.2%, 2%, and 1% were resistant to tetracyclines, macrolides, and clindamycin, respectively. ²⁰ Since the data we have about northern Israel are 10 years old, it was our responsibility to have an updated status of resistance rates of GAS to antimicrobials and make sure that our common practice of treatment is correct. The present study was conducted to assess the susceptibility to macrolides, clindamycin, and tetracyclines of GAS isolates collected in northern Israel and to compare the findings to the high antimicrobial susceptibility of GAS isolates reported in the same geographical region in 2004. ²⁵

Materials and Methods

Throat samples from outpatients consecutively received in a 2-month period of time (September–October 2011) at the regional laboratory of Emek Medical Center, which serves a population of one-half million in northern Israel, were included in the study. Samples were cultured on StrepSelect agar (Hy Laboratories, Rehovot, Israel) and incubated up to 48 hr in a 5% CO₂-enriched atmosphere. Beta-hemolytic colonies resembling streptococci were identified as GAS by susceptibility to bacitracin disks (0.1 units) and the Streptococcal Grouping Kit (Oxoid Ltd., Basingstoke, Hampshire, England). Minimal inhibitory concentration (MIC) for penicillin, erythromycin, azithromycin, clindamycin, and tetracycline was determined using the E-test (AB-Biodisk, Solna, Sweden) according to the manufacturer's instructions and interpreted using Clinical and Laboratory Standards Institute (CLSI) guidelines. Results within the resistant category were tested twice to avoid false-positive results. The D-test was performed for the detection of inducible resistance to clindamycin (methylation of the 23S ribosomal RNA encoded by the erm gene, also referred to as MLSB [macrolide, lincosamide, and type B streptogramin] resistance) by placing a 2-μg clindamycin disk 12 mm away from the edge of a 15-μg erythromycin disk on a blood-supplemented Mueller–Hinton agar plate. Following incubation, flattening of the clindamycin zone adjacent to the erythromycin disk (referred to as a D-zone) indicates inducible clindamycin resistance. ²²

Definitions of susceptible, resistant, and intermediate values are presented in Table 1.

Data were analyzed with SPSS software version 16.

Results

Three hundred GAS isolates were tested, 208 from pediatric patients (1–17 years of age, 102 females, 106 males) and 92 from adults (18–113 years of age, 72 females, 20 males). The overall mean age was 16.2 years (range 1–113, median 4.0) (Table 2).

A total of 295 (98%) isolates were susceptible to all the antibiotics tested. No resistance to penicillin was found. Only three isolates of GAS were found to be resistant to tetracycline, two from females (11 and 24 years old) and one from a male (12 years old). One isolate (0.33%) was resistant to both macrolides tested, erythromycin and azithromycin, from a 10-year-old girl. One isolate (0.33%) showed an intermediate susceptibility to clindamycin. No combined resistance was found. The D-test was negative for all 300 isolates, meaning that the MLSB resistance mechanism was not found. The mean MIC₉₀ and MIC₅₀ (in mcg/ml) were penicillin: 0.012 and 0.006, erythromycin: 0.125 and 0.094, azithromycin: 1.5 and 1.0, clindamycin: 0.125 and 0.094, and tetracycline: 0.094 and 0.064, respectively (Table 2).

Discussion

GAS is a leading human pathogen, causing mild to life-threatening infections, as well as postinfectious sequelae-like rheumatic fever. In Israel, there are few communities (e.g., ultraorthodox Jews) with high rates of invasive GAS diseases (up to 16 cases/100,000 per year^19,20). The goal of treating GAS pharyngitis is to shorten the length of disease and reduce the rate of complications; hence, a suitable initial empiric antimicrobial treatment is essential. Several studies published since our previous surveillance have also shown relatively low or decreased rates of antimicrobial resistance of GAS. In Korea, Germany, and Greece, pediatric populations were investigated. GAS isolates showed a significant decrease of resistance rates to erythromycin, clindamycin, and tetracycline.^6,12,28 Same low rates of resistance were found in additional studies on general populations in Serbia, ²¹ Turkey, ⁴ Belgium, ²⁹ and Taiwan. ¹⁰ In southern Europe, erythromycin, clindamycin, and tetracycline resistance was shown to have been decreased during 2005 to 2012. ¹⁸ None of these studies showed rates of antimicrobial resistance as low as in the current study.

On the other hand, several studies, particularly those performed on pediatric patients, have shown high levels of antimicrobial resistance of GAS in recent years. GAS isolates in China showed 95% resistance to macrolides and 92% resistance to tetracycline, ¹⁶ while the resistance rate to tetracycline was 61% in Brazil. ²³ As much as 78% resistance to clindamycin and erythromycin was reported in Latvia ³² and 33% resistance to erythromycin and 7% resistance to tetracycline in Spain. ²³ In Taiwan, very steep increases in resistance to erythromycin, clindamycin, and azithromycin were observed during 2011. ³ A recently published study in Lebanon showed an increased antimicrobial resistance compared to previous testing, in isolates from people of all ages; 9% resistance to clindamycin, 23% to erythromycin, and 37% to tetracycline. ¹¹

Frequent and inappropriate use of antibiotics in primary care health settings is considered to contribute to antimicrobial resistance worldwide. ³⁰ As demonstrated by recently published data collected from the pharmacy registries of Clalit Health Services ¹⁷ (the largest healthcare organization in Israel, representing 53% of the population nationwide and 65% in northern Israel), purchases of antibiotics, calculated according to defining daily doses (DDD/1,000 person), showed an increase by 11.3% in northern Israel, the region where the current study was conducted. Moreover, the use of macrolides doubled from 1998 to 2011. Despite the rise in total antimicrobial use, including macrolides in Northern Israel, the sensitivity of GAS to antimicrobials remains high. Comparing the sensitivity rates of GAS to macrolides in the two periods tested (2004 and 2011), even though the susceptibility rates remain low, a rise in the MIC₉₀ can be seen ²⁴ (Table 3).

Recently published data from the United States show a decrease in prescriptions for penicillin and an increase in prescriptions for macrolides, which are similar to the trends in Israel during the same period. In addition to the high susceptibility of GAS to penicillin,^9,16 the low cost and narrow spectrum of activity of penicillin make it the treatment of choice for GAS. ⁸ About 15% ¹³ of the population reports allergy to penicillin. Although real rates may be considerably lower,^1,25 many individuals are prescribed alternative antimicrobials without verification of allergy to penicillin. The most recently published IDSA guidelines on the management of GAS recommend that allergic patients should be treated with first-generation cephalosporin, clindamycin, or one of the macrolides: erythromycin, clarithromycin, or azithromycin and that tetracyclines should not be used due to the high prevalence of resistant strains. ²⁶ Although theoretically patients allergic to penicillin could be safely treated with first-generation cephalosporins (out of cases presenting major penicillin allergy), in our region, the treatment of such patients with macrolides is a very common practice. To avoid overuse of macrolides, azithromycin is a restricted drug needing authorization by Infectious Diseases Consultant. Therefore, periodic surveillance of antibiotic resistance to macrolides is essential when treating GAS. A nationwide surveillance study conducted in Israel (1996–1999) showed high sensitivity of GAS to penicillin (100%), erythromycin (98%), and clindamycin (99%), but increased resistance to tetracyclines (27.2%). Erythromycin resistance was associated with emm types 12 and 83, and tetracycline resistance with T types (3,3/13/B3624) and emm types (9,33,64,73,74,76,77,83). ²⁰ In our region, we found a very low resistance rate of GAS to tetracyclines, suggesting that this drug could still be safely used as an empiric treatment for patients who are allergic to penicillin.

Conclusions

This study showed that the antimicrobial susceptibility of GAS continued to be high in our geographical region, with findings similar to those obtained in a previous study conducted in 2004.

Despite the significant increase of DDD/1,000 patients in purchase of macrolides, resistance rates still remain low. The increasing trend of MIC₉₀ of azithromycin to GAS compared to our previous study should be watched and periodically monitored, but current recommendations of empiric treatment with this drug for proven GAS pharyngitis seem to be still appropriate in our region.

While our findings do not demonstrate a need for routine testing of the antimicrobial susceptibility of GAS pathogens, periodic surveillance is essential.