Interfakultäres Institut für Genetik und Funktionelle Genomforschung (MNF)
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Antibiotic resistance in pneumococci contributes to the high pneumococcal deaths in children. We assessed the molecular characteristics of multidrug-resistant (MDR) pneumococci isolated from healthy vaccinated children under five years of age in Cape Coast, Ghana. A total of 43 MDR isolates were selected from 151 pneumococcal strains obtained from nasopharyngeal carriage. All isolates were previously serotyped by multiplex PCR and Quellung reaction. Susceptibility testing was performed using either the E-test or disk diffusion method. Virulence and antibiotic resistance genes were identified by PCR. Molecular epidemiology was analyzed using multilocus sequence typing (MLST). Vaccine-serotypes 23F and 19F were predominant. The lytA and pavB virulence genes were present in all isolates, whiles 14–86% of the isolates carried pilus-islets 1 and 2, pcpA, and psrP genes. Penicillin, tetracycline, and cotrimoxazole resistance were evident in >90% of the isolates. The ermB, mefA, and tetM genes were detected in (n = 7, 16.3%), (n = 4, 9.3%) and (n = 43, 100%) of the isolates, respectively. However, >60% showed alteration in the pbp2b gene. MLST revealed five novel and six known sequence types (STs). ST156 (Spain9V-3) and ST802 were identified as international antibiotic-resistant clones. The emergence of international-MDR clones in Ghana requires continuous monitoring of the pneumococcus through a robust surveillance system.
Summary
Streptococcus pneumoniae (the pneumococcus), a bacterium belonging to the normal flora in the human respiratory tract, continues to be an important pathogen due to its contribution to morbidity and mortality among children, the elderly, and immunocompromised persons. Global estimates of pneumococcal deaths among children declined by 51% between 2000 and 2015. This achievement was mainly due to the introduction of pneumococcal conjugate vaccines (PCVs) in countries with the highest pneumococcal burden. Since May 2012, children in Ghana have been receiving PCV vaccination as part of routine immunization. The continuous monitoring of the pneumococcus after PCV introduction is essential to understand the changing epidemiology of the pathogen in the population.
This study therefore, aims to determine the (1) prevalence, serotypes, and sequence types of pneumococcal isolates, (2) antibiotic susceptibility patterns and the genetic basis for the antibiotic resistance among these pneumococcal isolates, and (3) prevalence of selected virulence genes that have been identified as potential vaccine candidates. Nasopharyngeal swabs were obtained from vaccinated children under five years of age in Cape Coast, Ghana. Six years after PCV implementation, we provide data on the epidemiology of pneumococcal strains circulating among children in Cape Coast Ghana. Standard microbiological and molecular techniques were used to identify and characterize the pneumococcal strains.
Overall, pneumococcal carriage prevalence was 29.4% (151/513). All participating children were fully vaccinated. Of the 26 different serotypes identified, the top five PCV13 serotypes (VT) were 6B, 23F, 19F, 3, 6A and non-PCV13 vaccine serotypes (NVT) were 23B, 13, 11A, 15B, and 34. PCV13 coverage was 38.4%, however, more than half of the isolates were NVT with a coverage rate of 61.6%. The isolates were highly susceptible to levofloxacin, ceftriaxone, vancomycin, and erythromycin. However, marked resistance to cotrimoxazole and tetracycline was observed. The reduction in penicillin resistance (35.8%) as compared to pre-vaccination data (45% - 63%) suggests an attributable effect from PCV13 vaccination. However, penicillin resistance was also detected in some NVT serotypes. Overall, 28.5% of the isolates resistant to three or more different classes of antibiotics were classified as multidrug-resistant (MDR). To analyze the genetic basis for resistance to penicillin, erythromycin and tetracycline, pbp2b, ermB, mefA, and tetM genes were amplified.
Thirty-eight (70%) out of the 54 penicillin-resistant isolates contained the pbp2b resistance gene. Out of the 11 erythromycin-resistant isolates, 7 (63.6) and 4 (36.4%) were positive for the ermB and mefA genes, respectively. The tetM gene was detected in 85 (98.8%) of the 86 tetracycline resistance isolates.
To determine the extent to which potential protein-based vaccines could be protective in Ghanaian children, we sought to determine the prevalence of selected virulence genes among the isolates. The lytA, pavB, and cpsA genes were present in all the carrier isolates. However, psrP, pcpA, pilus islet (PI) PI-1, and PI-2 were present in 62.7%, 87.5%, 11.8%, and 6.5% of the strains, respectively. The psrP and pcpA virulence genes were evenly distributed among all the serotypes. However, the pilus islets were detected in only seven serotypes namely 19F, 6B, 9V, 6A, 13, 11A, and 23B. Five serotype 19F isolates possessed both PI-1 and PI-2. Furthermore, the pilus islets were associated with multidrug resistance.
The predominant NVT serotype 23B and isolates resistant to ≥ 4 antibiotics were analysed by multilocus sequence typing (MLST). Nine known sequence types (STs) and 10 novel STs were identified. Seven out of the 10 new STs belonged to serotype 23B, while the remaining 3 were VTs 6B and 19F. A capsular switch was identified among isolates of ST802, which comprised of both serotype 23F and 19F. The majority of serotype 23B strains belonged to ST172. The ST172 is associated with serotype 23F and a single locus variant (SLV) of internationally disseminated clone ST338 (Colombia23F-26). Consequently, ST172 was characterised with marked antibiotic resistance and with traits of capsular switching. One serotype 6B strain was identified as a SLV of ST273 (Greece6B-22) while two serotype 9V strains belonged to the internationally disseminated clone ST156 (Spain9V-3).
In conclusion, this study showed a marginal decline in overall pneumococcal carriage prevalence, persistence of VTs despite the increase in NVTs, and the occurrence of serotype replacement and capsular switching. In addition, sequence types related to internationally disseminated clones are circulating in Ghana. With the high pcpA and psrP coverage detected,including these genes in protein-based vaccines could provide adequate protection for Ghanaian Children.