Entry Exclusion. The cell membrane of Gram-negative bacteria provides a barrier to entry of unwanted compounds into the cell. However, exchange of ions, nutrients and metabolic waste products requires access between the cell interior and the external environment, and thus the cell envelope contains a number of pores, channels, and transporters to allow entry of compounds necessary to the cell’s survival. Certain antimicrobial agents may take advantage of these entry points to gain access to targets within the cell interior. Mutations in the proteins that make up these channels, porins, and transporters may block entry of antimicrobials; this process is known as entry exclusion. Penicillin and tetracycline diffuse into the periplasmic space through porins. As previously mentioned, gonococci may express one of two porin alleles, PorB1A (PIA) or PorB1B (PIB) (87). Strains that express PorB1B are less sensitive to penicillin and tetracycline than those expressing PorB1A (95, 184, 231). Additionally, amino acid substitutions in loop 3 of PorB1B may increase resistance to penicillin and tetracycline (95, 184). The best studied of these mutations, collectively termed penB, are a glycine to lysine change at position 120, or glycine to aspartic acid and alanine to aspartic acid substitutions at positions 120 and 121, respectively (184). Interestingly, the effects of the penB mutation are only observable in strains bearing mutations in mtrR that over-express the MtrCDE multidrug efflux pump (discussed in Section V. D.), and mutations in penB cannot be selected without the presence of mutations in mtrR (231). Similarly, mutations may occur in PilQ, a doughnut-shaped structure in the outer membrane composed of 12 subunits used as a channel through which the pilus is secreted (66, 67). PilQ also appears to allow entry of antimicrobial agents, as null or missense mutations in PilQ have been shown to increase resistance of gonococci to a number of structurally diverse antimicrobials (49, 282). A glutamic acid to lysine change at position 1996, termed penC, prevents mutlimerization of PilQ, which has been shown to confer increased resistance to penicillin (282). Like penB mutations, however, the penC mutation is not sufficient to confer significant levels of penicillin resistance on its own; rather, it lends a further increase in resistance in strains carrying both the mtrR and penB mutations (282).
Entry Exclusion. The inability of an antimicrobial compound to penetrate Gram-negative bacteria due to the possession of an outer membrane provides the target microbe with a suitable first line of defense against invading antimicrobial agents and is known as entry exclusion. Some penetration of the outer membrane to obtain nutrients and compounds is necessary for gonococcal survival and occurs through the use of porins, channels, and transporters. These same conduits can be hijacked by antibiotics to serve as a way for antibacterial agents to enter into bacterial cells. In order to prevent penetration of the bacterial cell by antibiotics, mutations in porins, channels, and transporters have developed. For example, changes in porin alleles, termed penB, result in mutations on loop 3, leading to decreased antibiotic levels in the periplasmic space (68, 149). Interestingly, the gonococcal PilQ secretin, which forms a pore in the outer membrane, can also serve as a means by which antimicrobials penetrate the gonococcal periplasm. Alterations to pilQ that occur in combination with alterations to xxxX, mtrR, and penB result in increased resistance to both penicillin and tetracycline (219).
