We analyzed a total 213 samples of currency comprising 145 notes and 68 coins of various denominations, physical conditions and from different sources (Additional file 1). Two hundred (93.9%) samples were contaminated with different bacterial and fungal species. Majority of samples were contaminated with both bacteria and fungi (mixed contamination) than with only bacteria or fungi (Tables 1, 2 and 3). The isolation of both bacteria and fungi from currency as demonstrated by our study shows that currency could play an important role in the transmission of microbial agents in the community and thus presents a public health threat. Contamination of currency in our study is higher than reported in currencies in other developing countries like Nigeria, 52.5% [15], Nepal, 75% [26] and Saudi Arabia 72.3% [27] but comparable to the 94% reported in the United States [12] and lower than 100% recently reported in Ghana [20] and Pakistan [28]. These differences reflect differences in hygienic practices and handling of currency in different areas and also show that microbial contamination of currency is a global problem. Several behavioural practices in our study site may contribute to currency contamination: keeping money under body surfaces, improper washing of hands after using the toilet, wetting fingers with saliva when counting currency, coughing and sneezing on hands and handling currency, placement or storage of money on dirty surfaces during transactions and spraying during ceremonies. Generally notes (140/145, 96.6%) showed higher contamination than coins (60/68, 88.2%) (Tables 1 and 2). Paper currency (notes) has a cotton/linen composition and offers a large surface as breeding ground for microbes which can persist on it for longer periods. In addition, the surface of banknotes is not smooth, but irregular facilitating adherence by many different types of microbes. These factors ease colonization of notes more than coins. Coins are metallic in nature and depending upon the composition, they have been shown to possess antimicrobial activity. CFA (XAF) coins are nickel based. Coins composed of nickel alloy (nickel-brass and copper-nickel) to be more inhibitory to microbial colonization than coins of other metals [29]. In addition, the smaller surface area of coins provides a smaller surface for microbial colonization. These factors may account for our observation. Microbial growth was not detected in 16 new currency (‘mint’) samples used as control. It could be that these samples were contaminated by fastidious organisms and the media and/or culture conditions employed were inappropriate for their isolation. Since this money had not been in circulation we suggest usage and handling, as the possible causes of contamination of Franc CFA (XAF) in circulation.
The extent of contamination was found to be related to currency denomination (Table 1). There was a significant relationship in currency denomination and contamination (χ2 = 3.868, df = 1, P = 0.049) with lower denominations showing higher levels of contamination. Our findings are consistent with recent reports [20, 28, 30, 31]. Lower denominations pass through more hands for different daily transactions than higher denominations. The higher rate of exchange predisposes lower denominations to higher levels of contamination. Also, the tendency to mishandle currency is more among lower denominations than currency of higher values. Contrary to the results of Sabahat and Humaira [28] which did not suggest any one currency to be particularly susceptible to contamination, we observed the highest level of contamination (100%) among the 500 frs notes (Table 1). Our findings indicate that lower denominations harbor the greatest bulk of infectious agents particularly as they are exchanged more than higher denominations. However, no denomination was protected from contamination as we detected microbial growth in all denominations of notes and coins analyzed.
We also observed contamination of currency to be associated with the physical state of currency and source (Table 2 and 3). Contamination solely with bacteria (χ2 = 13.29, df = 1, P<0.001) and with both bacteria and fungi (χ2 = 12.42, df = 1, P<0.001) was significantly higher in dirty notes than in clean notes (Table 2) indicating a direct relationship between physical condition of the currency and contamination. Physical appearance of currency could serve as an indication of age of currency. Age has been suggested as another important factor that determines the occurrence of microorganisms on currency [32, 33].
All currency (100%) obtained from butchers and the hospital/clinic was contaminated. Highest contamination with both bacteria and fungi was observed in samples from food vendors (Table 3). Food either cooked or uncooked may contain microorganisms which can be transferred directly or indirectly through currency. Without proper hygiene practices, microorganisms can be transferred from currency to food. Thus, simultaneous handling of food and currency, a common practice among food vendors in study area could transfer pathogenic microorganisms to food. Handling of money between hand washing and food handling requires a repeat of the process of hand washing before handling of food. The detection of microbial contamination in currency from all sources is a great concern and indicates that contaminated currency could pose a health threat particularly to immune compromised individuals both in the hospital and in the community. Improved personal hygiene standards are highly solicited to reduce risk of infection from currency.
Pathogenic and potential bacterial species isolated from notes and coins included coagulase negative Staphylococcus (CoNS) (54.9%), Staphylococcus aureus (20.1%), Bacillus sp (13.6%), Corynebacterium sp (8.4%), Pseudomonas aeruginosa (8.4%), Escherichia coli (8.4%), Shigella dysenteriae (3.3%), Enterobacter aerogenes (0.9%) and Proteus mirabilis (0.4%) (Table 4). Our findings show that gram positive organisms are the predominant contaminants of CFA (XAF) circulating in study area. However, the isolation of both gram positive and gram-negative bacteria indicates that it could be serving as a reservoir of pathogenic organisms. There was no significant difference in the distribution of bacteria isolates on notes and coins of various denominations (Table 4). Coagulase-negative Staphylococcus, Staphylococcus aureus, E. coli and Bacillus sp were detected in all denominations of notes and coins.
The staphylococci (coagulase negative and S. aureus) were the predominant isolates in currency from all sources (Table 4), an indication of their ubiquitous nature. These organisms are normal flora of the skin and mucous membranes. Coagulase-negative staphylococci have long been regarded as non-pathogenic but their important role as pathogens and their increasing incidence have been recognized and studied in recent years. S. aureus are well-recognized pathogens [34]. Kumar et al.[21] have shown that S. aureus can survive on paper notes for eight days. Prolonged survival of this pathogen on currency permits transmission.
Bacillus species are spore-forming organisms that inhabit the soil and are ubiquitous in the environment. The isolation of Bacillus species from currency shows contamination with soil material. Bacillus species are generally perceived to be inconsequential. However previous studies [35] have highlighted the relevance of other Bacillus species as etiologic agents of non-gastrointestinal infections.
Corynebacteriun sp and Pseudomonas aeruginosa were other potentially pathogenic organisms recovered in currency. Pseudomonas aeruginosa is an important opportunistic pathogen causing a wide range of acute and chronic infections when introduced into areas devoid of normal defences [36].
Enteric pathogens E. coli and S. dysenteriae as well as potential pathogens P. mirabilis and E. aerogenges were isolated from samples analyzed. E. coli and S. dysenteriae were widely distributed in various denominations of currency (Table 4) than the other organisms. Their detection in currency is indicative of fecal contamination and poor sanitary conditions of the environment and personal hygiene practices of currency handlers. Thus, simultaneous handling of money and food should be discouraged unless proper hygiene is observed or food handling tools are used between the two processes. Our findings suggest CFA (XAF) as a vehicle for transmission of human pathogenic bacteria as money is in constant circulation.
Although E. coli is generally regarded as an opportunistic pathogen, studies [10, 18] have isolated the pathogenic strain O157:H7 from currency. This strain of E. coli organism has been shown to survive in currency for up to eleven days [10] making possible its transfer to humans. Sharma and Dhanashree [37] investigated the RAPD profiles of pathogens isolated from notes and coins and reported similar banding patterns of some isolates obtained from these sources confirming spread of pathogens through currency.
Fifteen species of fungi (Table 5) were isolated from both notes and coins from all sources. Aspergillus species predominated. Aspergillus species produce ochratoxins and aflatoxins. Aflatoxins have been shown to be carcinogenic [38]. In addition, inhalation of spores of their spores may cause aspergillosis making this organism a health threat. Penicillium can cause pneumonia [39]. Rhizopus is an agent for zygomycosis and eye infections [40]. Candida albicans, though a normal flora of humans causes a secondary infection in HIV/AIDS patients [41]. Its isolation from currency in a locality with high prevalence of HIV (as in other countries in the Central African sub-region) may present a challenge to the prevention of candidiasis in HIV infected individuals.
Bacteria isolated were generally susceptible to most of the antibiotics tested. Ceftriaxone, gentamicin, norfloxacin and ofloxacin were the most active drugs as they were effective against all isolates (100%) (Table 6). Other effective drugs included erythromycin (86.1%), cefotaxime (94.4%), cefuroxime (94.4%), and chloramphenicol (98.4%). Inactive drugs were amoxicillin, penicillin, ampicillin, cotrimoxazole and vancomycin as they showed low susceptibilities of 7.3%, 32%, 36.3%, 39.5% and 36.7% respectively. Resistant strains were found mostly among Pseudomonas aeruginosa, E. coli, Staphylococcus aureus and coagulase-negative Staphylococcus. Staphylococcus aureus and coagulase negative Staphylococcus were vancomycin resistant. Our findings have far-reaching public health implications because the CFA (XAF) is used in other countries in the Central African sub-region. Previous reports [42, 43] show that multidrug resistant Staphylococccus aureus and P. aeruginosa are widely distributed in study area. Our findings further confirm the ubiquity of drug resistant organisms in study environment with currency serving as a possible route for transmission. Since the CFA (XAF) franc is also used in other countries, there is the possibility of spread of these resistant pathogens to other countries, or it could be that these have been introduced from these countries. This implies that spread of these pathogens could be transboundary. To the best of our knowledge, this is the first study that has investigated the CFA (XAF) as a possible route for transmission of pathogenic organisms. Findings call for further investigation of the CFA (XAF) in various countries in the CEMAC zone to generate evidence-based findings for the urgent need for national and regional policies on currency handling and use. Also, molecular studies establishing a link between isolates from currency and those of clinical origin need to be carried out to ascertain the role of currency in transmission of pathogens.
Limitations
We did not include a control strain in antibiotic susceptibility testing. Also, the MIC of active drugs was not determined. Methods used to isolate microorganisms detected only culturable organisms. Molecular methods could have detected non-culturable or fastidious organisms. In addition, we did not investigate for other microorganisms such as protozoa or viruses which are likely to be common causes of gastrointestinal or respiratory illnesses in the community.