The emergence of drug resistance globally, is currently presenting a large and growing problem in infections that account for  most  of  Africa’s  disease  burden,  including tuberculosis (TB), respiratory and diarrheal diseases (Okon et al., 2013; Padmavathy et al., 2014; Sangeeth et al., 2014). In addition to the multi-drug resistance problem, the nosocomial infections (healthcare-associated infections) are associated with high mortality. This has necessitated a need for a continuous search and development of novel antimicrobial substances from different biological sources to minimize the threat of further antimicrobial resistance (Padmavathy et al., 2014; Shah et al., 2014).

Mushrooms have been recognized as functional foods and as a source for the development of medicines and nutraceuticals (Alves et al., 2012).  Basidiomycetes, to which mushrooms belong, are a group of higher fungi with distinctive fruiting bodies and reproductive structures. Some mushrooms are edible, while others are extremely poisonous. There are about 140 000 species of mushrooms and of these, only 22 000 are known, while only a small percentage (5%) has been investigated (Faridur et al., 2010). Mushrooms have been prescribed for treatment of various human diseases such as gastro-intestinal disorder, bleeding, high blood pressure and various microbial infections (Akyuz, 2010; Gbolagade and Fasidi, 2005). Many varieties of mushrooms have been identified as major sources of biologically active natural products, such as oxalic acid and sulphated lentinan from Lentinula edodes, triterpenes and ganodermin, an antifungal protein, both from Ganoderma lucidum, polysaccharopeptides from Coriolus versicolor, water-soluble lignins from Inonotus obliquus and velutin, a ribosome inactivating protein from Flammulina velutipes (Chaudhary and Tripathy, 2015; Collins and Ng, 1997; Lindequist, 2005; Moon and Lo, 2014; Wang and Ng, 2006). These compounds may be sources of natural antibiotics and may have immunomodulatory, cardio-vascular, antifibrotic, anti-inflammatory, antidiabetic, antioxidant, antiviral, antimicrobial and antitumor properties (Alves et al., 2012; Gan et al., 2013; Geethangili et al., 2013; Ramesh and Pattar, 2010; Tehrani et al., 2012; Wang and Ng, 2004).

In recent years, a number of studies were conducted in various countries to determine the potential therapeutic properties of mushrooms. The reported bioactivities from mushrooms include antibacterial, antifungal, antioxidant and antiviral properties (Padmavathy et al., 2014; Reis et al., 2011). The species Cantherellus, Lentinus, Russula, Agaricus and Pleurotus are examples of mushrooms that have shown antimicrobial properties against Bacillus species, Enterococcus, Streptococcus, Staphylococcus and Micrococcus species (Alves et al., 2012; Khan and Tania, 2012; Pushpa and Purushothama, 2010). Crude organic and aqueous extracts from Ganoderma have been reported to inhibit in vitro growth of Escherichia coli, Staphylococcus   aureus,    Bacillus    cereus,    Neisseria meningitides, Alcaligenes faecalis and Proteus vulgaris, bacteria known to cause wound infections, intestinal and urinary-genital tract infections and skin infections (Shikongo et al., 2013). The European Ganoderma has been reported to inhibit growth of most bacteria especially methillicin-resistant S. aureas (Linderquist et al., 2005).

Zimbabwe is rich in mushroom diversity. However, the potential of mushrooms as source of new drugs is still largely unexplored (Sharp, 2011, 2014). Despite many studies on potential therapeutic properties of different mushroom species globally, little or no work has been carried out on the antimicrobial activities of mushrooms in Zimbabwe. In addition, there are several wild edible species of mushrooms which are yet to be exploited in Zimbabwe. Thus, the main aim of this work was to investigate the antimicrobial potential of different extracts of ten selected wild edible and non-edible mushrooms found in Zimbabwe.

Statistical analysis

Experimental values are given as means ± standard deviation (SD). Graph-pad prism was used to analyse the data. Statistical significance was determined by both one and two way variance analysis (ANOVA). All experiments were carried out in triplicate.

In similar studies carried out by Quereshi et al. (2010), methanol, ethanol, acetone and cold water extracts of G. lucidum from India showed antimicrobial activity against the S. aureus, S. typhi and E. coli bacterial culture collections. From this study, the methanol extract showed no inhibition to S. aureus and E. coli, while the ethanol and acetone extracts inhibited growth of both E. coli and S. typhi but did not inhibit growth of S. aureus. The water extracts showed no inhibition to all the bacteria tested. Ethanol  extracts  of  G.  lucidum  from  Turkey  inhibited growth of E. coli while the methanol extract showed no inhibition (Celik et al., 2014). In another study, acetone and ethanol extracts of Cantharellus cibarius collected in Turkey, exhibited antibacterial activity against E. coli and S. aureus but showed no inhibition against S. typhi (Dulger et al., 2004). Results of a study in Nigeria showed that methanol and ethanol extracts of Cantharellus cibarius from Nigeria inhibited E. coli and S. typhi growth but showed no inhibition against S. aureus and S. pneumoniae (Aina et al., 2012). Similarly, results obtained from this study show that methanol, ethanol and acetone extracts of the three Cantharellus species studied exhibited various degrees of inhibition against the four bacteria tested. This shows that different species of mushrooms exhibit different antimicrobial activity due to a number of factors such as the presence of different antimicrobial components, type of the extracting medium, geographical location of the mushroom and the type of organism being tested.

Extracts obtained from ethanol gave the highest number of bacterial growth inhibition (33), followed by acetone (31) and methanol (28). In addition, ethanolic extracts showed the strongest antibacterial activity (8-14 mm) among the five extracts against the bacterial strains, followed by methanol and acetone. Water extracts exhibited the lowest number of antibacterial activity, despite having the extract with the highest inhibitory effect. This indicates, that the active compounds from the mushrooms studied which inhibit the growth of susceptible bacteria, may dissolve better in the organic solvents than in the aqueous solvents. These results are consistent with already reported literature that extracts from organic solvents give more consistent antimicrobial activity than water extracts (Kamra and Bhatt, 2012; Tiwari et al., 2011). It is interesting to note that, although cold water and hot water extractions gave highest values of total phenolic compounds in Table 2, these had the least effect on most bacteria. This shows that the antibacterial activity in the mushroom extracts depends not only on the presence of phenolic compounds but also on the presence of various secondary metabolites. Ethanol, acetone and methanol extracts were all effective against all the four bacteria indicating the broad spectrum of antibacterial activity of the extracts. However, Gram negative bacteria were slightly more susceptible to the extracts than Gram positive bacteria (52 and 46 extracts, respectively). Many antibiotics are designed to attack the integrity of the cell wall by preventing cell wall synthesis, therefore killing the cell. Although, all bacteria have an inner cell wall, Gram negative bacteria have a unique outer membrane which prevents certain drugs and antibiotics from penetrating the cell. Thus, antibiotics that affect the cell wall will impair Gram positive bacteria and not Gram negative bacteria. The results obtained in this study suggest that the antibacterial extracts may act by affecting not just the cell wall, but other cell growth mechanisms like protein synthesis, bacterial DNA replication and transcription. Among the four bacteria tested, S. typhi was the most susceptible bacteria as indicated by its highest number of inhibitions as well as the highest number of most potent extracts in the 8-14 mm diameter range. A decline in the number of multi-drug resistant clinical isolates (S. typhi) has been reported (Madhulika et al., 2004). Thus, the study shows that the S. typhi isolate studied, may be a phage type that is susceptible to most antibiotics.

The antibacterial activity of the ethanolic, methanolic and acetone extracts against E. coli, S. typhi, S. aureus and S. pneumoniae is of great importance in the human healthcare system. S. pneumoniae is the most common cause of community acquired pneumonia (CAP) in children while E. coli accounts for more than 70% of the infections of the urinary tract worldwide  (Blossom  et  al., 2006; Sangeeth et al., 2014). S. typhi is the cause of typhoid fever, which was recently epidemic in Zimbabwe. S. aureus is the most common cause of bacterial infections and abscesses of skin, joints and bones (Stanely et al., 2013). Resistance to antibiotics has been reported in S. aureus, S. pneumoniae, S. typhi and E. coli (Blossom et al., 2006; Okonko et al., 2009; Rowe et al., 1997; Sangeeth et al., 2014; Stanely et al., 2013). All the bacterial strains used were clinical isolates from individuals in Zimbabwe. E. coli and S. aureus are mostly encountered in urinary tract infections while isolated cases of S. typhi are common. Thus, the antibacterial activity found in the mushroom extracts can be further investigated for future use in the development of therapeutic agents to treat infections caused by these bacteria.

Wild edible and non-edible mushrooms can be used as agents in the development of new drugs for bacterial infections. This study indicated that the antibacterial effects of mushrooms vary depending on the type of mushroom, the solvent medium used and the type of organism tested. C. heinemannianus and C. symoensii had the highest effect on inhibition of bacteria as indicated by the different extracts showing high inhibitory properties ranging from 8-14 mm [15.4 (8) each], followed by G. lucidum [13.5% (7)], while Boletus edulis, Coprinus sp. and Trametes strumosa had the least effect [5.8% (3) each]. Extracts obtained from ethanol were the most effective tested against bacteria (36.5%), followed by methanol (30.8%) and acetone (30.8%) and lastly, aqueous extracts (1.9%). Thus, of the five solvents tested, ethanol, methanol and acetone were determined to be the solvents of choice for isolation of antibacterial compounds from the majority of mushrooms studied. However, identification of the phyto-constituents responsible for the antibacterial activity is required for large commercial production.

The authors declare that there is no conflict of interest.

This study was funded by the Fogarty International Center Training Grant (MTRCBSA – (2D43TW001587-10A2) and the University of Zimbabwe Research Board. The authors acknowledge Cimas Medical Aid Society and Lancet laboratories for providing the bacterial strains and the Biochemistry Department for the laboratory facility used in this study. Mrs   Pawandiwa’s support and provision of technical assistance during the study is greatly appreciated.