This study investigated molecular characteristics, antimicrobial resistance patterns and capsular types of S. pneumoniae isolated from blood samples in Tehran, Iran. Of the 44 isolates, 73%, 68% and 16% were resistant to erythromycin, tetracycline and penicillin, respectively which were similar to levels reported for these antibiotics among pneumococci isolated in Iran (71%, 67%, 17%) and the rate of resistance in penicillin (16–28%) and erythromycin (67%) in some Asian countries such as Malaysia and Thailand [18, 19].
In our study, the major mechanism conferring resistance to macrolide antibiotics, was the constitutive phenotype that 43% of isolates showed this resistance and 52% of the isolates harboring ermB gene. In agreement with this finding, other studies in the Turkey, China and Taiwan showed that the ermB gene was the most frequent macrolide resistance determinant. while the most common resistance gene in the Canada and United States was mefA/E gene [20, 21]. In this study the rate of co resistance related to ermB and mefA/E gens was found in 23% of isolates which accordance with a report from Turkey (20%) [20]. As for frequency of double resistance to erythromycin and clindamycin has been spread globally. The high prevalence of dual resistance in S. pneumoniae was found in South Korea and South Africa [5, 21, 22].
The majority of macrolide-resistant strains 75% (24/32) were also resistant to tetracycline. This association is due to the insertion of ermB into composite transposons of the Tn916 family that contain tetM gene [7, 20, 23]. While, the existence of unexpressed tetM genes in tetracycline sensitive isolates showed that transposons of the Tn916 family may be more widespread in S. pneumonia than expected to firmly associated with resistant tetracycline [23, 24].
This study showed that the most common capsular types detected in 64% of the pneumococcal isolates was 6A/B, 19A, 15A, 23F. These results are partly similar to those from most Asian countries such as Japan (6B, 23F, and 19F), China (14, 19A, 23F) and Turkey (1, 19A, 19F, 6A/6B, 14) [25–27]. Generally, a small number of capsular types (1 ،6A/B, 23F, 19A, 19F, 7F، 9V and 14) are the most prevalent capsular types in IPD isolates [1, 2, 4].
Capsular type 19A is frequent among MDR isolates which has already been described in many non-vaccinated regions such as Korea [28]. Researchers have formerly exhibited that the spread of MDR capsular types 19A isolates is due to antibiotic misuse in developing countries [29]. In our country, irrational use of antibiotics has contributed to the emergence of MDR isolates
In current study, more than 70% (5/7) of capsular type 19A isolates were MDR and showed resistance to erythromycin (majority MIC ≥ 256 µg/ mL), tetracycline (MIC ≥ 8 µg/ mL), clindamycin, and trimethoprim-sulfamethoxazole. One isolate of capsular type 19A in addition of mention antibiotics showed relatively high resistance to penicillin which carried Tn2009. As for, all (6/6) of capsular type 23F isolates and 50% (2/4) of capsular type 11A, 15B/C (1/2) isolates were MDR (high level MIC for erythromycin and tetracycline) as capsular type 19A isolates but 50% of 11A, 15B/C and 23F were resistance penicillin (MIC ≥ 32–48 µg/ mL).
Despite the importance of capsular type as an invasive determinant, other virulence determinants were also associated with invasive isolates [30]. As our results, the majority of the isolates contained the virulence determinants that probably indicate the essential of virulence determinants in the ability of an isolate to cause invasive disease [30]. Interestingly, pspA gene was encoded by the most pneumococcal isolates which only detected in 41% of our isolates. This is in accordance with the study suggesting that probable limitation of detection by conventional PCR and confirmed this hypothesis by a quantitative PCR assay at high level detection [31].
As for capsular type is assumed to be more important than genotype in the ability of an isolate led to invasive disease but also underline the role of genetic background in invasion [30, 32, 33]. Since pneumococcal isolates with diverse MLST profiles have showed various pathogenicity potential [32]. According to the other studies suggesting high capsular type and genetic diversities in IPD isolates [10, 25, 33, 34], there was important diversity among our isolates base on capsular types and different MLST profiles. In pneumococcal isolates, one of the important factor to selective pressure is use of antibiotics [34, 35], so the antibiotic selection pressure may be led to different genetic diversity of IPD isolates that observed in this study [34, 35]. However, causing agents associated with genetic diversity should be further studied.
Reliable and comprehensive data regarding antimicrobial resistance and genetic characteristics, S. pneumoniae are scarce, in Iran. This prompted our research. The main limitations of this study are rather small number of invasive isolates and the pneumococcal isolates were collected from a few hospitals.