Antimicrobial Resistance of Streptococcus pneumoniae Clinical Serotypes between 2017 and 2022 in Crete, Greece

Streptococcus pneumoniae is an important human pathogen causing a wide range of diseases manifesting either as non-invasive infections such as bacterial pneumonia, otitis media, and sinusitis or invasive infections such as meningitis, bacteremia, and sepsis [1]. Pneumococcal diseases are associated with high morbidity and mortality rates, especially in young children, the elderly, and individuals with comorbidities [1, 2]. According to the European Centre for Diseases Control and Prevention report for 2018, the incidence of invasive pneumococcal disease (IPD) was found to be 18.7 cases per 100,000 population in adults aged 65 years and above [3]. IPD can result in high mortality rates despite antimicrobial therapy ranging from 15 - 20% in adults to 30 - 40% in the elderly [4].

Over the last several decades penicillin has been the drug of choice for the treatment of pneumococcal infections. The first clinical isolate of S. pneumoniae penicillin-non-susceptible was detected in 1967 [5]. Since then the increasing global consumption and misuse of antibiotics contributed to increasing non-susceptibility rates in S. pneumoniae towards different classes of antibiotics. The high rates of drug resistance and the spread of multidrug-resistant (MDR) strains of S. pneumoniae constitute serious public health problem worldwide. In 2014, in a World Health Organization report on antibiotic resistance, pneumococcus was listed as one of the nine bacteria of international concern [6]. European data from 2018 report large intercountry variations in non-susceptibility of invasive S. pneumoniae to penicillin and macrolides with overall percentages of 18% and 18.6%, respectively [3]

Vaccination is an important prevention strategy against pneumococcal infections. Implementation of pneumococcal vaccination reduces the prevalence of resistance by reducing the overall burden of disease and by targeting the most resistant serotypes. Consequently, the need for antibiotic use is reduced, preventing the development of resistance to antimicrobials. In Greece, the 23-valent pneumococcal polysaccharide vaccine (PPV23) was licensed in 1998 for immunization of individuals aged ≥60 years and since 2011 it is also recommended for adults 19 - 50 years with a wide variety of medical conditions that increase the risk of IPD. PCV7 was introduced in the pediatric national immunization program in 2006 and was replaced by PCV13 in June 2010. In December 2011 the indication of PCV13 was expanded to adults ≥50 years of age [7]. Since 2015, PCV13 has been recommended for adults aged 19 - 64 years and for adults ≥65 years of age sequentially with PPSV23 [8]. In 2019, the vaccination rates of adults for PCV13 and PPSV23 were 49.5% and 23.5%, respectively [9].

In Crete, surveillance of serotypes and antimicrobial susceptibility of S. pneumoniae was initiated in our laboratory in 1997 [10]. The current study shows the results of a 6-year survey of serotypes and antimicrobial resistance involving adult S. pneumoniae isolates causing invasive and non-invasive infections following the implementation of PCV13.

In the present study, serotypes 11A, 35B, 3, 15A, 25F, 19A, and 35F were more common, and an increase in rates of serotypes 35B (5.2% vs. 10.3%) and 11A (8.9% vs. 10.3%) was detected when compared with the rates reported between 2009 and 2016 in Crete [14]. PCV13 serotypes in adult pneumococcal infections decreased from 37.8% over the period 2009 - 2016 to 26.7% over the years 2019 - 2022. This overall decline was largely driven by decreases in serotypes 7F and 19A. A significant decrease was also observed in PPSV23 serotypes (73.3% vs. 57.8%), which was offset by a substantial increase in NVTs (17.0% vs. 37.1%).

In the present study, 35B and 15A are among the most common serotypes, that are not included among the serotypes targeted by both PCV13 and PPSV23. After the introduction of PCV13 an increase in serotypes 35B and 15A was observed in IPD cases in both adults and children in several countries worldwide, including USA, Canada, UK, and Japan [15, 16, 17, 18]. In this study, the isolates belonging to serotype 35B were more common in non-invasive diseases and both serotypes, 15A and 35B showed a trend toward MDR (55.6% and 16.7%, respectively). Similarly in Japan serotypes 15A and 35B were most frequently detected in non-invasive isolates associated with higher resistance and MDR [19].

Serotype 11A was the most prevalent serotype during the study period, more commonly associated with non-IPD cases in the elderly. In recent years, serotype 11A is a prevalent serotype in Europe, accounting for 2 - 7% IPD and 9 - 10% non-IPD [20]. Several studies have shown that 11A has low invasive potential, and is associated with antibiotic non-susceptibility and increased risk of death [21, 22, 23, 24].

Regarding serotype 3, we did not observe any change in the incidence of this serotype, which remained stable over the last 12 years after PCV13 introduction. This is in line with observations reported by other investigators in countries that introduced PCV13 in infant vaccination [15, 25, 26]. PCV13 is less immunogenic for serotype 3 compared to other serotypes resulting in reduced both direct protection to those vaccinated and indirect protection through high carriage rates and increased S. pneumoniae transmission [27].

The introduction of PCV13 has been associated with reduction in pneumococcal diseases in both children and adults and reduction of the antimicrobial resistance [28]. In our region, penicillin resistance decreased from 14.3% to 7.4% during the early post-vaccine period, and 12.1% in the late post-vaccine period, compared to pre-vaccine period [14, 29]. On the contrary, the prevalence of intermediately susceptible strains increased from 23.6% in the pre-vaccine period to 31% in the late post-vaccine period, increasing the penicillin non-susceptible S. pneumoniae (PNSP) by 5.2%. Similarly, a significant increase in penicillin resistance was also observed from 1.9% over the period 2017 - 2019 to 20.3% over the years 2020 - 2022. The increasing rates in PNSP was in part due to increases in pneumococcal diseases caused by serotypes not included in PCV13, and the persistence of serotype 19A [20].

Despite the high penicillin non-susceptibility rates, resistance to amoxicillin was low. The high susceptibility rates to amoxicillin and its low impact on resistance selection, combined with the fact that amoxicillin achieves high respiratory tissue concentrations, provide a strong evidence supporting European guidelines for the management of adult community-acquired lower respiratory tract infections, who recommend amoxicillin as first-line therapy [30]. Similarly, rates of non-susceptibility to 3rd-generation cephalosporins are very low. This is important because ceftriaxone is the antibiotic of choice for treatment of meningitis [31].

Resistance to macrolides was high (40.5%), and significantly increased over the two three-year periods. The increased macrolide resistance is likely due to the extensive use of macrolides, principally for community-acquired respiratory tract infections. Even higher rates of S. pneumoniae macrolide resistance have been reported in China (92.0%), Taiwan (89.1%), and Korea (80.3%) [32, 33, 34].

Resistance to fluoroquinolones increased by 2.9% compared to the pre-vaccine period. However, the level of resistance to fluoroquinolones remains low despite their widespread use. In our observation, all fluoroquinolone-resistant isolates belonged to serotypes not included in PCV13.

The incidence rate of MDR decreased from 58.1% in the pre-PCV13 period to 31% in the late post-PCV13 period, and was more common among serotypes 19A, 15A, 25F, 11A, and 35B. Higher rates of resistance and MDR were observed among non-IPD than IPD, as previously reported by other investigators [35, 36].

In conclusion, the increasing levels of resistance in S. pneumoniae in Crete, highlight the need for continuous surveillance on antimicrobial resistance trends to help selection of appropriate antimicrobials and develop strategies to reduce antibiotic resistance. However, this study is conducted on a single geographic area, so the present findings are not representative of the epidemiology of the whole country. There may be differences in serotype distribution and antimicrobial resistance among different regions of the country. Given the diversity of serotype epidemiology in different age groups and different geographical areas, monitoring of serotype evolution is necessary for development of next generation PCVs with a wider serotype coverage, in order to decrease the burden of pneumococcal diseases.