Phase variation, the stochastic, high frequency, reversible alteration of gene expression, serves as a mutational mechanism capable of pre-empting bacterial populations for environmental change, including the invasion of the environment by predators such as bacteriophage. Preparation of the bacterial population occurs through the constant generation of heterogeneous populations consisting of distinct phenotypes able to cope with multiple environmental selections. In Haemophilus influenzae Rd KW20, ON/OFF phase variable expression of the lipooligosaccharide extension encoding gene lic2A has been shown to alter the bacterial host’s susceptibility to bacteriophage HP1c1, with lic2A OFF cells immune from HP1c1 infection. In the ON state, lic2A encodes a glycosyltransferase responsible for the addition of a galactose residue to the surface exposed lipooligosaccharide (LOS), facilitating bacteriophage adsorption. Phase variation of lic2A is achieved through stochastic, high frequency, and ultimately reversible, expansion and contraction of a tetranucleotide (5’-CAAT-3’) repeat tract within the open reading frame of lic2A, resulting in frame shifts that either prevent or reinstate expression of lic2A.

Here we simulate experimentally how varying degrees of heterogeneity for lic2A expression within the bacterial population facilitates survival of the H. influenzae population upon predation by bacteriophage HP1c1. These levels of heterogeneity are portrayals of different theoretic levels of selection pressure for either lic2A ON or lic2A OFF expression, as while lic2A in the OFF state gives bacteriophage resistance, lic2A ON expression aids the bacterial population in immune evasion. In addition we investigate the degree to which varying levels of bacteriophage selection pressure can alter H. influenzae population composition, giving rise to the heterogeneity for lic2A expression.