Harmful bacteria are often associated with the toxins they use on their hosts. It is generally believed that the greater the toxicity the more severe is the disease. A new study, though, has shown that bacterial toxicity and disease severity are not necessarily linked in this way.
The new study involves the “major human pathogen” Staphylococcus aureus which is considered a superbug, that is, a bacterial species that has grown resistant to antibiotics.
Infections caused by S. aureus living in the nose and on the skin can be only mild, but they are lethal if they escape into blood vessels causing a condition called bacteraemia. Why does this bacterial species cause death in one case and no harm in another? This difference in severity has been the main focus of the new research.
Two collections of isolated S. aureus were analysed: one came from a patient before and after developing bacteraemia while the second one came from many patients (some from patients’ noses and others from their bloodstream after they developed bacteraemia).
In both cases, the bacteria from the blood which would cause the severest disease, were found to be the least toxic. Why is this so?
According to the scientists, secreting toxins was tedious for the bacteria since they had to spend much energy to do so. Furthermore, the blood is full of immune system agents ready to combat antigens, and is, therefore, an environment putting much pressure on the bacteria. The researchers hence concluded that the energy needed to secrete toxins is too high for the bacteria to be able to adapt to stressful conditions in the bloodstream, implying that the less toxic bacteria are the ones to reach the blood causing the most severe infections.
This interpretation of the results is in stark contrast to what was generally believed by scientists: that toxins are critical to the bacteria’s ability to cause disease.