2.2.4.2. Antibiotic Selection
Another disadvantage with the LAB-S. pneumoniae vaccines is the use of antibiotic-resistance markers, which are considered unacceptable in live vaccines due to the potential for antibiotics in the final product and the possible contamination of the environment with recombinant drug-resistant bacterial strains. The regulatory agencies also prohibit the usage of antibiotics in vaccine formula. Two antibiotics are commonly used in the LAB vectors. The first one is erythromycin. Erythromycin can inhibit the protein synthesis by binding the 50s subunit of the bacterial 70s rRNA complex. Most plasmids used in LAB vaccines have an erythromycin-resistance selection marker. This antibiotic is necessary to select the recombinant plasmid. LAB strains with recombinant plasmid are grown with erythromycin prior to immunization to maintain the plasmid. Adding the antibiotics not only increases the costs of the final product, but also raises the concern about the plasmid stability. The most used pTREX vectors have poor segregational stability in the absence of antibiotic selection [172]. LAB could lose the recombinant plasmid in vivo and lead to compromised immune responses. To conquer this problem, the use of the balanced-lethal strategy could be attempted with LAB vaccines. Another is the nisin. Nisin is a polycyclic lantibiotic produced by L. lactis to eliminate other competing Gram-positive bacteria. It is commonly used as a safe food preservative against bacteria, yeast, and molds. Nisin can bind to lipid, dissipate the membrane potential, induce efflux of cytoplasmic components and inhibit bacterial cell growth