Davis Kimberly M, Akinbi Henry T, Standish Alistair J, Weiser Jeffrey N
Department of Microbiology, University of Pennsylvania School of Medicine, Philadelphia, PA, USA.
PLoS Pathog. 2008 Dec;4(12):e1000241. doi: 10.1371/journal.ppat.1000241. Epub 2008 Dec 12.
The abundance of lysozyme on mucosal surfaces suggests that successful colonizers must be able to evade its antimicrobial effects. Lysozyme has a muramidase activity that hydrolyzes bacterial peptidoglycan and a non-muramidase activity attributable to its function as a cationic antimicrobial peptide. Two enzymes (PgdA, a N-acetylglucosamine deacetylase, and Adr, an O-acetyl transferase) that modify different sites on the peptidoglycan of Streptococcus pneumoniae have been implicated in its resistance to lysozyme in vitro. Here we show that the antimicrobial effect of human lysozyme is due to its muramidase activity and that both peptidoglycan modifications are required for full resistance by pneumococci. To examine the contribution of lysozyme and peptidoglycan modifications during colonization of the upper respiratory tract, competition experiments were performed with wild-type and pgdAadr mutant pneumococci in lysozyme M-sufficient (LysM(+/+)) and -deficient (LysM(-/-)) mice. The wild-type strain out-competed the double mutant in LysM(+/+), but not LysM(-/-) mice, indicating the importance of resistance to the muramidase activity of lysozyme during mucosal colonization. In contrast, strains containing single mutations in either pgdA or adr prevailed over the wild-type strain in both LysM(+/+) and LysM(-/-) mice. Our findings demonstrate that individual peptidoglycan modifications diminish fitness during colonization. The competitive advantage of wild-type pneumococci in LysM(+/+) but not LysM(-/-) mice suggests that the combination of peptidoglycan modifications reduces overall fitness, but that this is outweighed by the benefits of resistance to the peptidoglycan degrading activity of lysozyme.
黏膜表面溶菌酶的大量存在表明，成功的定殖菌必须能够规避其抗菌作用。溶菌酶具有水解细菌肽聚糖的溶菌酶活性以及因其作为阳离子抗菌肽的功能而产生的非溶菌酶活性。两种修饰肺炎链球菌肽聚糖不同位点的酶（PgdA，一种N - 乙酰葡糖胺脱乙酰酶，和Adr，一种O - 乙酰转移酶）在体外已被证明与肺炎链球菌对溶菌酶的抗性有关。在此我们表明，人溶菌酶的抗菌作用归因于其溶菌酶活性，并且肽聚糖的两种修饰对于肺炎球菌的完全抗性都是必需的。为了研究溶菌酶和肽聚糖修饰在上呼吸道定殖过程中的作用，我们在溶菌酶M充足（LysM(+/+)）和缺乏（LysM(-/-)）的小鼠中用野生型和pgdAadr突变型肺炎球菌进行了竞争实验。野生型菌株在LysM(+/+)小鼠中比双突变体更具竞争力，但在LysM(-/-)小鼠中并非如此，这表明在黏膜定殖过程中抵抗溶菌酶的溶菌酶活性很重要。相比之下，在pgdA或adr中含有单个突变的菌株在LysM(+/+)和LysM(-/-)小鼠中都比野生型菌株更具优势。我们的研究结果表明，单个肽聚糖修饰会降低定殖过程中的适应性。野生型肺炎球菌在LysM(+/+)而非LysM(-/-)小鼠中的竞争优势表明，肽聚糖修饰的组合会降低整体适应性，但这被抵抗溶菌酶肽聚糖降解活性的益处所抵消。
