New and effective antibiotics are crucial in this current surge of multi-drug resistant bacterial infections which have rendered many of the currently available antibiotics useless. Natural products have served and continue to provide useful lead compounds for development into chemotherapeutic agents. Aquatic microorganisms have emerged as a source of diverse chemical compounds which have not been adequately studied for chemotherapeutic application. Our results have revealed 27 (23%) antibiotic producing microorganism out of 119 isolates recovered from both marine and fresh water sources in Ghana and this is the first report of this kind of study in the West African sub-region. Many reports have been made of such studies elsewhere. For example Ivanova et al
] reported that out of the 491 bacteria isolated from different marine sources, 26% of the isolates were active. Zheng et al
] also reported that 8 out of 29 strains, representing 28% of the isolates considered in their study produced antimicrobial activity against at least one of their test microorganisms. Brandelli et al
] also recorded 70% of active isolates from the Amazon Basin whilst O’Brien et al
] recorded as low as 0.29% (13 out of 4496) of active microbes from soil samples collected at different location in the Antarctica.
The comparatively high number of antibiotic producers recorded in our study can be partly attributed to the nature of our water bodies: they are usually highly polluted with all kinds of waste materials; from domestic and industrial wastewater discharges, mining runoff, agro-chemicals and other sources
] and river wiwi, Lake Bosomtwe and the Gulf of Guinea at Duakor Sea Beach where the samples were collected are no exceptions. To survive and maintain their niche under these harsh conditions therefore, the aquatic microorganisms need defense mechanisms and for some, antimicrobially active metabolite production could be one of such mechanisms. The differences among the detection rates reported in literature strongly depend on the isolation and assay procedures, test organisms, type of media used, as well as the sources of bacterial isolates
]. In our study, only those isolates producing extracellular antibiotics were detected, hence very huge numbers could be recorded if our procedures include microorganisms producing intracellular antibiotics since they will only secrete their antibiotics into media in the presence of competition, to antagonise other organisms for survival
Isolate MAI2 which was identified as a strain of Pseudomonas aeruginosa
, exhibited the highest antibacterial activity and produced perhaps, moderately thermo-stable antibacterial metabolites, shown by exhibition of antibacterial activity when the metabolites solution was exposed to temperatures up to 100°C but destroyed at 121°C for 15 min. This result is in contrast to those reported by Preetha et al.
] who reported that the antimicrobial agent produced by Pseudomonas
species MCCB was stable after autoclaving at 121°C for 20 min even though there was a significant reduction in activity. Uzair et al
] also reported the thermal stability of an antimicrobial agent produced by Pseudomonas aeruginosa
at a temperature of 121°C for 20 minutes. However, Roitman et al
] showed that variations in the fermentation medium often results in changes in the composition of the antibiotics produced. The differences in the thermal stability of the antimicrobial agents produced in this study as compared to other studies may therefore be due to differences in some nutritional and or physical factors which led to the production of metabolites that are thermolabile at temperatures beyond 100°C.
Our results also showed that nine days incubation period was optimum for maximum antibacterial activity by MAI2, an indication of maximum antibiotic production, after which there was no significant increase. Several other factors influence production of secondary metabolites by microorganisms, the most important one being the composition of the fermentation medium
]. Sole et al
] noted that glucose can be used as a source for bacterial growth while repressing the production of secondary metabolites. The isolate (MAI2) utilised glycerol and starch best for maximum production of the antimicrobial metabolites.
Nitrogen is very vital in the synthesis of enzymes involved in primary and secondary metabolism
]. Therefore depending on the biosynthetic pathways involved, nitrogen sources may affect antibiotic formation. Shapiro
] noted that the type of nitrogen source (organic or inorganic) plays a role in the synthesis of secondary metabolites. Charyulu and Gnanamani
] reported that Pseudomonas aeruginosa
MTCC 5210 utilized organic nitrogen source for better yield of antimicrobial metabolites than the inorganic sources. These observations are consistent with the findings of this study as asparagine was better used for antibiotic production by MAI2 than the inorganic nitrogen sources (sodium and potassium nitrates and the ammonium salts) employed.
Generally, the intracellular pH of most microorganisms is maintained near neutrality regardless of the pH in the outside medium
]. However as the proton gradient across the cytoplasmic membrane increases, the cells commit more of their resources towards maintaining the desired intracellular pH
], thus changes in external pH affect many cellular processes such as growth and the regulation of the biosynthesis of secondary metabolites
]. The highest activity of the antimicrobial metabolite by the strain was at pH 7. This result agrees with a study carried out by Charyulu and Gnanamani
] who reported maximum production of metabolite by Pseudomonas aeruginosa
MTCC 5210 at pH 7.
Isolate MAI2 exhibited antimicrobial activity against both gram-positive and gram-negative microorganisms as well as C. albicans, indicating that the metabolites have a broad antimicrobial spectrum.
The seven components observed in the TLC analysis of the extract points to the fact that organisms can produce more than one antimicrobial agent to provide themselves with survival competition superiority. Further work is ongoing in our laboratory to isolate and test the various components of the extract. It is hoped that these components when isolated into pure constituents can serve as leads for the development of novel and potent antibiotics as well as resistant reversing compounds
] which may be useful in combination therapies as exemplified by clavulanic acid in AugmentinR
The extract is bacteriostatic in its mode of action since there were revivable cells of the test organisms in the wells in which inhibition was observed. Bacteriostatic agents like the β- lactams have been of great value in the treatment of bacterial infections including endocarditis, meningitis, and osteomyelitis
]. Other bacteriostatic agents such as the lincosamides (example clindamycin) have been shown to completely inhibit the toxic shock syndrome toxin-1 production by Staph. aureus
] and toxin production in both streptococci and staphylococci
]. These reports suggest that the active constituents MAI2 crude extract have the potential of being efficacious in the treatment of various infections.