In a test I was asked why bacteria are insensitive to snake toxins.
Is it their membrane that provides a barrier to the toxins? Or do snake poisons have specific targets and thus cannot bind to bacteria?
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Many snake poisons target specific proteins not present in unicellular organisms.
The question is admittedly broad but the idea behind this question is pretty much what you indicate in your post - many venoms target specific proteins and do not simply destroy their target by, e.g., disrupting gross cellular structure (like alcohol does for example). Instead, they target specific molecules that are essential for the survival of their prey.
Snake toxins can be categorized according to the organ systems they target, namely :
Central nervous system toxins are carried by elapid snakes like cobras, kraits and the taipan. Typical targets are the nicotinic acetylcholine receptor and the muscarinic acetylcholine receptor. Blockade of these receptors at the neuromuscular junctions resulting in death by asphyxiation. Acetylcholine receptors are not present in bacteria.
Cardiovascular toxins are pretty diverse and include things like angiotensin-converting enzyme inhibitors (leading to a drop in blood pressure) and glycosaminoglycans (the sulphated carbohydrate moieties that occur abundantly in cells of cardiovascular tissues) binding proteins that lead to cardiotoxicity. Again, the targets are specific molecules involved in heart function and hormones, stuff not present in bacteria.
Muscular toxins include those that bind specifically to the sarcoplasmic reticulum of muscles or interfere with specific second messenger systems messing up muscular function. Again, quite specific targets not present in bacteria.
Lastly, typical vascular system toxins include anti-coagulants such as protein C activators and inhibitors of prothrombin complex formation. Again, specific targets.
Venoms from vipers contain high amount of proteolytic enzymes (serine proteases). Many of them act by cleaving fibrinogen and thereby causing blood clot (ref). There is a likelihood that some of these proteases may affect other proteins also. In a study conducted by Bottrall et al. (2010), it was shown that snake venoms do have a general proteolytic activity. The best among tested was the venom of the viper Bitis arietans. However, its activity was significantly lower than the positive control which consisted mainly of trypsin and proteinase-K.
So the venom may have a little effect on the membrane proteins of bacteria. But it might require extended treatment by venom to digest these proteins compared to that by stronger proteases like proteinase-K.
However, there are indeed reports on antibacterial activity of snake venoms (Stiles et al., 1991; Perumal Samy et al., 2007; Charvat et al., 2018). But the enzymes responsible for the antibacterial activity are phospholipase A2 and L-amino acid oxidase, and not the proteases.