Resistance of bacteria against antimicrobial agents has always been an immense
burden to healthcare worldwide. High resistance of Staphylococcus aureus
against ciprofloxacin, amoxicillin and chloramphenicol has been documented in
a study (Nafeesa et al., 2001). The problem has
led to wider search for antimicrobial agents from other sources against the
antibiotics resistance bacterial strains. Among these new sources are plant
extracts, which have shown promising activities against methicillin resistant
S. aureus (MRSA), vancomycin resistant S. aureus (VRSA) and vancomycin
intermediate S. aureus (VISA) (Dadgar et al.,
2006; Zakaria et al., 2007). Snake venom is
another new source of antimicrobial agents that we will describe in this study.
Snake venoms constitute a diverse range of proteins that include neurotoxins, enzymes and peptides. Consequently, the idea of harvesting proteins from snake venoms to be developed as commercial, therapeutic products has not been new.
Given its vast potential as a source of therapeutics, it is, therefore, unsurprising
that snake venoms have also been investigated for antimicrobial components.
Despite heavy colonization of pathogenic bacteria in the oral cavity of the
snake, snake bite victims are not frequently observed to suffer from wound infection.
This observation led to the theory that snake venom may have antimicrobial activity
(Talan et al., 1991). While, the significance
of antimicrobial component in snake venoms is yet to be fully elicited, it has
been hypothesized that these components in the oral secretions of the snake
have been developed under evolutionary pressure as a defense mechanism of the
snake against the microorganisms on its prey (Shivik, 2006).
Among some of the common antimicrobial components that have been isolated from
snake venoms are the enzymes L-amino oxidase (LAAO) and phospholipase A2
(PLA2). The LAAO purified from Pseudechis australis showed
significant antimicrobial activity against different strains of Aeromonas
(Stiles et al., 1991). PLA2 purified
from Crotalus durissus terrificus and Daboia russelli siamensis
have shown antibacterial activity against the Gram-negative bacteria Burkholderia
pseudomallei (Samy et al., 2006). PLA2
isolated from Agkistrodon halys also showed significant inhibition against
Staphylococcus aureus, Proteus vulgaris and Proteus mirabilis
(Samy et al., 2008).
Apart from enzymatic proteins, antimicrobial peptides have also been purified
in recent studies. Cathelicidins isolated from Bungarus fasciatus and
Ophiophagus hannah have reported potent antimicrobial activity against
many strains of Gram negative bacteria (Wang et al.,
2008; Chen et al., 2009). A small peptide
vgf-1, isolated from Naja atra, has shown to have antimycobacterial activity
(Xie et al., 2003). Omwaprin, purified from Oxyuranus
microlepidotus, is reported to show antimicrobial activity against Gram
positive bacteria such as Staphylococcus wagneri and Bacillus megaterium
(Nair et al., 2007).
In this study, we describe the preliminary screening of 11 snake venoms for antibacterial activity against three different strains of Staphylococcus aureus and two Gram negative bacteria, Pseudomonas aeruginosa and Escherichia coli. Similar screening focusing on antimicrobial property has not been attempted previously among indigenous Malaysian snakes.
MATERIALS AND METHODS
Venoms: All venoms used were of common venomous snakes in Malaysia, obtained from the snake farm in Sungai Batu Pahat, Perlis, Malaysia, in the year of 2009. The venoms were all, freeze-dried and stored in -20°C.
Bacterial strains: The bacterial strains used in the antibacterial screening assays were Staphylococcus aureus ATCC25923, Staphylococcus aureus ATCC29213, Methicillin-resistant Staphylococcus aureus (MRSA) ATCC43300, Pseudomonas aeruginosa ATCC25873 and Escherichia coli ATCC25922. The bacteria were streaked onto Mueller-Hinton (MH) agar plates, incubated overnight for growth and stored at 4°C.
Bacterial inoculum: Bacterial inoculum that would be used for antibacterial screening assays was prepared using the standard method of log-phase growth. A single-unit colony was picked from the bacterial culture plates and added to 10 mL of Mueller-Hinton (MH) broth. The inoculated MH broth was incubated at 37°C until a colony-forming unit (CFU) mL-1 value of 105. This was confirmed by measuring the absorbance value of the inoculum with the spectrophotometer (Shimadzu) to give A600=0.1.
Antibacterial screening assays: Lyophilized venoms (1 mg mL-1) were prepared in deionized water. Three milliliter of bacterial inoculum (cfu mL-1 = 105, A600 = 0.1), was spread onto an agar plate and left standing for 3 min. Excessive inoculum was poured away and the agar plate was left standing for 3 min again. Using a modified hole-plate method, 4 mm diameter wells were made onto the agar using an immunodiffusion cutter (Nair et al., 2007). Thirty microliter of each dissolved venom sample was loaded into each well. Vancomycin and methicillin (30 μg mL-1) were used as drug controls. The bacterial plates were incubated at 37°C for 18 h and the inhibition zones were measured in millimetre diameters.
Minimum inhibitory concentration: Minimum Inhibitory Concentration (MIC) was determined using a modified method described by Nair et al., 2007. The two venoms which showed the highest diameter of inhibition zones were chosen for MIC determination. Crude venom of concentration 1 mg mL-1 was serially diluted in the range of 0.5, 0.25, 0.125, 0.0625 and 0.03125 mg mL-1 using cation-adjusted MH broth. Three milliliter of bacterial inoculum was spread onto 30 mL of MH agar as described above for the antibacterial screening assays. Six wells of diameter 4 mm were made onto the agar plate. Each well was loaded with 30 μL of one concentration of the diluted venom. The plates were incubated at 37°C for 18 h and MIC was defined as the lowest concentration of venom which gave a detectable inhibition zone.
Table 1 shows the antibacterial activity of 11 crude venoms tested against five different strains of bacteria, three different strains of S. aureus and two different strains of Gram negative bacteria. From this preliminary screening, the Gram negative bacteria were found to be resistant against all the crude venoms as no inhibition zone was exhibited in the antibacterial assay.
The antibacterial activities of the crude venom against the Gram positive bacteria were found to be more significant among the Viperidae species. Crude venoms from all four Viperidae species demonstrated significant inhibition zones between 6.6-12.5 mm. Calloselasma rhodostoma showed the largest inhibition zones between of 10.2-12.5 mm.
Among the crude venoms from the six Elapidae species, only three exhibited antibacterial activity. The inhibition zones of Bungarus candidus and Bungarus fasciatus demonstrated against the three strains of S. aureus were only between 6.3-8.0 mm. Ophiophagus hannah was the only exception among the Elapidae species, exhibiting large inhibition zones between 9.5-11.5 mm.
|| Antibacterial activity of 11 crude venoms against five strains
of bacteria tested using hole-plate method
|The values are presented as Mean±SD (n = 4). The inhibition
zones were measured after 18 h of incubation at 37°C and the inhibition
zone diameters include the 4 mm diameters of the wells containing either
30 μL of crude venom (1 mg mL-1) or antibiotic (30 μg
mL-1). Bacterial inoculum of 105 cfu mL-1
(A600 = 0.1) is added per plate. SA: Staphylococcus aureus;
PA: Pseudomonas aeruginosa; EC: Escherichia coli. -: No activity
||Antibacterial assays of crude venom using the hole-plate method.
(a) SA ATCC29213, (b) SA ATCC25923 and (c) SA ATCC43300. SA: S. aureus;
KC: O. hannah (King Cobra); CR: C. rhodostoma; TS: T. sumatranus;
BC: B. candidus; BF: B. fasciatus; NN: N. kaouthia;
Van, Vancomycin; Met, Methicillin. A vancomycin disc (30 μg mL-1)
was also used against SA ATCC29213, as shown in (1a) in comparison with
diluted vancomycin solution (30 μg mL-1). The diluted vancomycin
showed higher inhibition diameter than the vancomycin disc
The venom of the single Hydrophiidae species screened in this study exhibits
no antibacterial activity.
The crude venoms also have lower antibacterial activity as compared to that of the conventional antibiotics, as shown by the larger inhibition zones of vancomycin and methicillin.
Figure 1a-c showed the antibacterial assays
of the various crude venoms tested against the three strains of S. aureus
ATCC29213, ATCC25923 and ATCC43300 using the hole-plate method.
|| MIC values of C. rhodostoma, O. hannah and
|MIC was determined as the lowest concentration of venom which
gave a detectable inhibition zone after 18 h of incubation at 37°C.
The venoms were diluted in the range of 1, 0.5, 0.25, 0.125, 0.0625 and
0.03125 mg mL-1. Each concentration was added to one well. Bacterial
inoculum of 105 cfu mL-1 (A600 = 0.1) is
added per plate. SA, Staphylococcus aureus
In all three assays shown, it could be seen that venoms from Calloselasma
rhodostoma and Ophiophagus hannah demonstrated clear inhibition zones
that were significantly larger than the other crude venoms screened.
Since the crude venoms from Calloselasma rhodostoma and Ophiophagus hannah demonstrated the most significant inhibition zones against the three strains of S. aureus, including the MRSA strain, these two venoms were selected for MIC determination. Table 2 shows the MIC values of Calloselasma rhodostoma and Ophiophagus hannah. The MIC values of both venoms were found to be within the range of 125-250 μg mL-1. When tested against S. aureus ATCC25923 and ATCC43300, Calloselasma rhodostoma demonstrated lower MIC as compared to Ophiophagus hannah. The MIC values of these two venoms were found to be much higher than that of the antibiotics.
One of the main patterns that can be identified in this preliminary screening
is that the Viperidae venoms exhibited larger inhibition zones than Elapidae
venoms, with the exception of Ophiophagus hannah. This pattern is very
similar to the pattern found when enzymatic properties of Viperidae and Elapidae
venoms were compared, which showed that venoms of Viperidae species had higher
enzymatic activities than venoms of Elapidae species, with the exception of
Ophiophagus hannah venom, an Elapidae venom that showed high enzymatic
activities (Kocholaty et al., 1971). The results
of this preliminary screening, therefore, highly suggest that the antibacterial
activity of snake venoms is due to enzymatic components. This is also in tandem
with other findings, which have shown that snake venom antimicrobial activity
is due to enzymes such as PLA2 (Samy et al.,
Another pattern that can be identified from this preliminary screening is the susceptibility of the three strains of Gram-positive S. aureus to the crude venoms and the resistance of the two strains of Gram negative bacteria against the venoms.
Similar patterns of dose-dependent species specific susceptibility and resistance
against Gram negative have also been noted in omwaprin, a peptide isolated from
the venom of Oxyuranus microlepidotus. Omwaprin is effective against
Gram positive bacteria such as Bacillus megaterium and S. wagneri,
but shows resistance when tested against Gram negative bacteria and other Gram
positive bacteria such as B. thuringiensis, S. aureus and Streptococcus
clavuligerus (Nair et al., 2007). The resistance against Gram negative
bacteria could possibly be due the outer membrane of the bacteria. The outer
membrane of Gram negative bacteria has lipopolysaccharides (LPS). Charges on
the LPS molecule can affect the uptake of antimicrobial peptide and such similar
mechanism might have resulted in the resistance we observed among the Gram negative
bacteria tested in this screening (Devine and Hancock, 2002).
It has also been observed that Gram negative bacteria, when treated with EDTA
that disrupts the outer membrane, become susceptible to antibacterial agents
such as bronchochin (Gao et al., 1999).
Nevertheless, this preliminary screening is by no means representative and
conclusive of the resistance of Gram negative bacteria against snake venoms,
as the number of bacterial strains used is very minimal. Moreover, PLA2
and cathelicidin previously isolated from other snake venoms were found to be
effective against Gram negative bacteria such as Burkholderia pseudomallei
and E. coli (Samy et al., 2006; Wang
et al., 2008), contradicting our findings from this screening. Species
specific susceptibility also could not be identified from this screening as
only one type of Gram positive bacterium has been used.
While it has been observed with electron microscopy that the antibacterial
mechanism of snake venoms involves cell surface membrane blebbing and subsequent
cell contents leakage (Nair et al., 2007; Samy
et al., 2008) the exact mechanism of the cell membrane disruption
leading to cell death is still very much unclear. Snake venom antimicrobial
mechanism is complex and is affected by factors such as amino acid sequence,
net charge of the protein, three-dimensional structure, bacterial membrane composition
and salinity of the environment (Nair et al., 2007).
Hence, the different proteins/peptides from different species of snakes can
have varied mechanisms of cell membrane disruption, resulting in the differences
of susceptibility among the different bacterial strains.
While the MIC values obtained from this screening is many times higher than
that of the conventional antibiotics, it has been shown in a comparison study
that purified LAAO exhibited significantly higher inhibition than crude venoms
(Samy et al., 2007). The inhibition exhibited
by fractions obtained from each stage of purification also increases (Samy
et al., 2008).
In summary, we have successfully established the potential of antimicrobial activity using snake venom and have also identified two promising venoms, Calloselasma rhodostoma and Ophiophagus hannah, which show the highest inhibition. Further study is in the progress to purify the active antimicrobial proteins/peptides from these two venoms, as well as to screen a larger number of bacteria to characterize the susceptibility of these two venoms more accurately.
Authors would like to thank Ministry of Science, Technology and Innovation (MOSTI), Malaysia for funding under EScience grant: 02-02-10-SF0033 and Monash University Sunway Campus for funding under Internal grant: MED2010-INI(MG)-001-JV.