DNA vaccination by mecA sequence evokes an antibacterial immune response against methicillin-resistant Staphylococcus aureus.

More than 90% of methicillin-resistant Staphylococcus aureus (MRSA) isolates produce a penicillin-binding protein PBP2' (or PBP2a) with low affinity for beta-lactam antibiotics. PBP2' is encoded by the mecA gene, a foreign gene integrated into the chromosome of methicillin-susceptible S. aureus (MSSA). DNA vaccination by injection of transgene-expressing plasmids has been demonstrated to elicit an immune response against transgene-encoded protein. We hypothesized that the application of DNA vaccination with the mecA sequence would elicit protective immunity against MRSA. This immunity was evoked by injection of a mecA-expressing plasmid into BALB/c mice. Anti-PBP2' antibody was detected in the sera obtained from the DNA-vaccinated mice. These sera produced a five-fold increase in phagocytosis of MRSA compared with sera from mice treated with control plasmid. However, there was no difference in phagocytosis of MSSA among these groups. In addition, the in-vivo antibacterial effect of DNA vaccination was demonstrated in mice infected with MRSA. Eight days after iv inoculation of 10(8) cfu of MRSA into mice, the number of bacteria in the kidneys obtained from mice vaccinated with mecA-expressing plasmid (1.48 +/- 0.27 x 10(5) cfu/mg kidney; n = 18) was significantly lower than that from mice vaccinated with negative control plasmid (3.59 +/- 0.57 x 10(5) cfu/mg kidney; n = 17) (P < 0.02) or that from sham-treated mice (3. 43 +/- 0.66 x 10(5) cfu/mg kidney; n = 9) (P < 0.02). Interestingly, PBP2' was found in both the bacterial membrane fraction and the supernatant, thus being accessible to serum antibodies. Together these observations indicate that PBP2' or the mecA sequence may be eligible as a candidate molecule for vaccination against MRSA.