Efforts to identify EPS-I mutants that produce a short Vsa protein have been unsuccessful. Thus, it cannot be ascertained whether EPS-I is required for efficient adherence when Vsa is short. No mutants that lack Vsa protein have been identified in our robust transposon library, suggesting that these proteins are essential (Dybvig et al., 2010). Past studies have concluded that M. pulmonis cells producing a short Vsa are sensitive to lysis by complement, leading to the hypothesis that the Vsa proteins form a protective
shield (Simmons et al., 2004). Cultures inoculated with mycoplasmas that produce a short Vsa protein have a longer lag phase than cultures inoculated with cells producing a long Vsa but have a rapid growth rate in exponential phase and reach a high titer (Dybvig et al., 1989), suggesting an initial toxicity that is labile and from which the long Vsa can protect. Ferroptosis inhibitor cancer EPS-I may also have a role in protection, rendering EPS-I mutants with a short Vsa protein difficult to isolate. Because it was intriguing that EPS-I promoted cytoadherence, but inhibited biofilm formation, hemadsorption assays were utilized as an additional approach to examine interactions among the mycoplasma and host cells. Hemadsorption has often been used as an indicator for adherence to pulmonary epithelia in multiple species of mycoplasma (Hasselbring et al., 2005). The utter lack of hemadsorption that was observed for mycoplasmas
resulted in phenotypic differences. For example, the Vsa isotype has no known association with any tissue tropism (Gumulak-Smith et al., 2001; Denison et al., 2005). The EPS-I mutants are currently available only in the VsaG background, thus nothing can be said about the role of EPS-I in hemadsorption. However, the remaining Vsa isotypes A, I, and H all exhibit hemadsorption profiles in concurrence with previously published data, with short Vsa-producing strains exhibiting significantly greater hemadsorption than long Vsa-producing strains (Simmons & Dybvig, 2003). Bacterial pathogens generally produce multiple adhesins, and colonization is a complex process. The adhesins and receptors involved in the colonization of the murine host by M. pulmonis are unknown as are the precise roles of the Vsa proteins and the EPS-I polysaccharide. Cells producing a long Vsa protein or lacking EPS-I may retain the ability to colonize animals because although cytoadherence is reduced, it is not eliminated. Mycoplasmal structures resembling the towers of biofilms that develop on glass or plastic surfaces have been observed ex vivo and in vivo on the mouse trachea (Simmons & Dybvig, 2009). Thus, although mutants lacking EPS-I cytoadhere poorly, their enhanced ability to form a biofilm may be a contributing factor to their ability to efficiently colonize animals.