e-Science : Oct 2016 Issue 18
STAYIN’ ALIVE Bacterial resistance to an antibiotic can evolve under the selective pressure of the antibiotic, either directly against the antibiotic or through a modification in the antibiotic-target interaction. These molecular mechanisms for antibiotic resistance can be transferred amongst a bacterial population on mobile genetic elements. The race between the discovery of new antibiotics and antibiotic resistance has narrowed and it has now been some time since a new antibiotic was introduced into the market. However, in 2015 an innovative approach to screen 10,000 bacterial strains (including previously unculturable bacteria) led to a new antibiotic, a cell wall inhibitor called teixobactin. Experiments spectacularly showed there was no detectable resistance against teixobactin that developed. Chronic and relapsing infections There are several human pathologies resulting from infection that notoriously are difficult to treat. This is especially true for many infections by the bacterial pathogen S. aureus and its infections of bone and joints (chronic osteomyelitis), nasal infection (chronic rhinosinusitis) and infections of heart valves (infective endocarditis). In each of these cases, S. aureus is not cleared by antibiotics and surgical intervention is required, which itself does not always remove the infection. In other cases, the antimicrobial treatment seems to have worked but the infection and the disease reappear after a period of latency. In both these circumstances the bacteria have an antibiotic tolerance but not a resistance. RESOURCES Antibiotic resistance is measured using the minimum inhibitory concentration (MIC) of a bacterial strain for the antibiotic/s. Antibiotic tolerance is much more complex. It describes a bacterial ability to survive high concentrations of an antibiotic, which can sometimes be much higher than the MIC. It usually refers to a sub-population within a non-resistant bacterial strain. This tolerance can be driven through different mechanisms but it is usually generated by a switch in the bacterial lifestyle to a dormant or quasi-dormant state. A bacterial cell that is not growing (or growing very slowly) is not susceptible to antibiotics. The bacterial cells in this small portion of an antibiotic-susceptible bacterial culture are referred to as persister cells and they can tolerate high concentrations of antibiotics and thereby remain present after a seemingly successful treatment. A clinical sample of normal S. aureus, or golden staph, with the characteristic golden colonies.
May 2016 Issue 17