In this study, we assessed the microbial composition of chronically infected wounds in rural Ghana. The bacterial spectrum seen was in line with findings from previous studies conducted in another region of Ghana, in Ethiopia and in Tanzania [1, 11–15].
Altogether, Gram-negative bacteria dominated quantitatively. Enterobacteriaceae, Pseudomonas aeruginosa and Staphylococcus aureus were the most frequently isolated organisms. Previous assessments both from America and Europe as well as from Africa confirm Staphylococcus aureus and Pseudomonas aeruginosa as the leading pathogens isolated from chronic wounds. Both species express virulence factors and surface proteins negatively affecting wound healing [1, 9, 16, 17]. Pathogenicity in chronic wounds is further increased by co-infections of Staphylococcus aureus and Pseudomonas aeruginosa [16].
Biofilms caused by Pseudomonas aeruginosa play important roles by maintaining chronic wound infections thus preventing healing. Such biofilms are also known to show high adherence to biological surfaces, a phenomenon which limits the sensitivity of swabbing-based sampling with subsequent culture-based diagnostic approaches in comparison to molecular biological approaches [18]. Consequently, it is likely that the true prevalence of Pseudomonas aeruginosa was even higher.
Although Pseudomonas aeruginosa followed by Staphylococcus aureus were the most common bacteria identified on species level, the most frequent isolates were Enterobacteriaceae. The estimation of their clinical relevance is much more difficult. Although it is generally accepted that Enterobacteriaceae like Escherichia coli and others might play a role in wound infections [17, 19]. inhabitants of tropical or subtropical climate zones were shown to have high colonization rates of skin and mucous membranes with Gram-negative rod-shaped bacteria [20]. High frequencies were also reported from patients, students, and health-care workers in Madagascar [21]. This makes the discrimination between wound colonization and wound infection challenging. From the less frequently isolated bacteria, beta-hemolytic streptococci including Streptococcus pyogenes are highly likely to be etiologically relevant for the assessed chronic wound infections [22], while enterococci are frequent colonizers of uncertain clinical relevance in wounds [23]. In mono- and polymicrobial infections, Pseudomonas aeruginosa was the most common bacterial specie. Others, including Enterobacteriaceae and enterococci were rarely observed in cases in which only one microorganism was isolated, stressing the likelihood that they are probably contamination flora or minor components of polymicrobial infections.
The observed antimicrobial resistance patterns are of interest. The recorded 29% MRSA is in discordance with a Ghanaian study on wound infections from 2014, where no MRSA were identified from infected wounds [14]. This suggests a considerable increase of MRSA within less than a decade; although it has to be considered that selection processes in chronic wounds after repeated antimicrobial therapy are likely. High frequencies of 3rd generation cephalosporin and fluoroquinolone resistance but absence of carbapenem resistance in Enterobacteriaceae isolated from wounds in Ghana has, in contrast, been described previously [14]. The increase of fluoroquinolone resistance in Ghana has recently been described [24]. significantly reducing the value of this group of antibiotics for treatment of severe infections. In addition, we found one carbapenem-resistant Pseudomonas aeruginosa. Carbapenem-resistance is still comparatively rare in Ghana: 2.9% in Gram-negative rod-shaped bacteria, predominantly in Pseudomonas aeruginosa and Acinetobacter baumannii [25]. In summary, the observed resistance patterns make any rational antimicrobial therapy challenging, indicating a need for routine diagnostics including antibiotic susceptibility testing. However, antibiotic resistance seen in bacterial isolates from chronic wounds is likely to overestimate the true resistance rate in clinical isolates, because selection processes due to repeated attempts of antibiotic treatment have to be considered as likely.
In regards to fungi isolated from chronic wounds, information on the immune status of the patients was not assessed. Hence potential etiological relevance remains speculative. In general, fungal infections have been seen to cause skin infection including mycetoma that may manifest as chronic wounds after trauma [26]. Detection of causative fungal pathogens requires prolonged culture on fungus-specific media or demonstration of fungal elements in tissues. The performance of broad-range fungal PCR to amplify fungal DNA for the diagnosis of these infections has not been evaluated [27]. Our results suggest that the applied approach may be more likely to amplify DNA of yeasts that may be normal skin flora or microbial communities of chronic wounds. Specific PCR assays may be more sensitive for the amplification of specific fungal pathogens such as Madurella mycetomatis [26]. Limitations of this study include geographic restrictions of sampling, reducing the representativeness of the study for the whole of Ghana. In addition, swabbing of neighboring sites of intact skin to better differentiate infection from colonization [28] was not performed. Also, more sensitive molecular diagnostic methods were not applied, making it likely that several components of mixed infections will have been missed [18]. Also, despite their potential involvement in chronic wound infections, no anaerobic bacterial culture was performed.