Investigation of the influence of phase variable restriction modification systems and lipooligosaccharide epitopes on resistance of Haemophilus influenzae to infection by bacteriophage
thesisposted on 10.07.2017, 09:19 by Christopher Jason Richard Turkington
The evolution of ON/OFF switching phase variable loci is presumed to have arisen due to the need for bacterial populations to cope with uncertain environments where selection fluctuates between opposing pressures. One such selection is believed to be the presence of bacteriophage. Bacteriophage can influence bacterial evolution by forcing the development of bacteriophage resistance mechanisms within bacterial populations. However, resistance mechanisms can often come at a cost, such as reduced survival against the immune responses. As such, phase variation may circumnavigate this cost by generating heterogeneous populations containing both resistant and sensitive phenotypes. This study aimed to investigate the two known phase variable bacteriophage resistance genes in Haemophilus influenzae hsdM and lic2A, which encodes the methyltransferase of a type I restriction modification system and a lipooligosacharide biosynthesis associated glycosyltransferase respectively. Analysis of the diversity of repeat tract lengths and ON/OFF states of these genes across H. influenza genomes within GenBank revealed that while lic2A was ON in the majority of strains, hsdM was OFF. Thus lic2A may be consistently beneficial while the hsdM benefit is transient. Analysis of samples from patients with COPD, showed that lic2A was ON in the majority of samples, while hsdM may also be ON state in a number of samples. Although a resistance phenotype could be observed for lic2A, no resistance could be observed for hsdM. Therefore, the dynamics of bacteriophage spread through populations heterogeneous for lic2A was investigated. The heterogeneous populations generated by phase variation reduced bacteriophage dispersal through bacterial populations. This mechanism may also allow bacterial populations to adjust their heterogeneity levels to control bacteriophage densities. The heterogeneity may further create diverse bacteriophage densities across the bacterial macropopulation through resistant populations acting as a barrier. The results demonstrate the potential for phase variation to aid in the survival of bacterial populations against bacteriophage predation.