Division of Infectious Diseases, Boston Children's Hospital, Boston, Massachusetts, United States of America.
Department of Pediatrics, Harvard Medical School, Boston, Massachusetts, United States of America.
PLoS One. 2022 Sep 28;17(9):e0270697. doi: 10.1371/journal.pone.0270697. eCollection 2022.
Nicotinamide adenine dinucleotide (NAD+) is an essential co-factor for cellular metabolism and serves as a substrate in enzymatic processes. NAD+ is produced by de novo synthesis or salvage pathways in nearly all bacterial species. Haemophilus influenzae lacks the capacity for de novo synthesis, so it is dependent on import of NAD+ from the external environment or salvage biosynthetic pathways for recycling of NAD+ precursors and breakdown products. However, the actual sources of NAD+ utilized by H. influenzae in the respiratory tract are not well defined. In this study, we found that a variety of bacteria, including species found in the upper airway of humans, released NAD+ that was readily detectable in extracellular culture fluid, and which supported growth of H. influenzae in vitro. By contrast, certain strains of Streptococcus pyogenes (group A streptococcus or GAS) inhibited growth of H. influenzae in vitro by secreting NAD+-glycohydrolase (NADase), which degraded extracellular NAD+. Conversely, GAS strains that lacked enzymatically active NADase released extracellular NAD+, which could support H. influenzae growth. Our results suggest that many bacterial species, including normal flora of the upper airway, release NAD+ into the environment. GAS is distinctive in its ability to both release and degrade NAD+. Thus, colonization of the airway with H. influenzae may be promoted or restricted by co-colonization with GAS in a strain-specific manner that depends, respectively, on release of NAD+ or secretion of active NADase. We suggest that, in addition to its role as a cytotoxin for host cells, NADase may serve a separate function by restricting growth of H. influenzae in the human respiratory tract.
烟酰胺腺嘌呤二核苷酸（NAD+）是细胞代谢的必需辅酶，也是酶促过程中的底物。几乎所有细菌物种都通过从头合成或补救途径产生 NAD+。流感嗜血杆菌缺乏从头合成的能力，因此依赖于从外部环境中导入 NAD+或补救生物合成途径来回收 NAD+前体和分解产物。然而，流感嗜血杆菌在呼吸道中利用的 NAD+的实际来源尚未得到很好的定义。在这项研究中，我们发现多种细菌，包括在上呼吸道中发现的物种，释放了可在细胞外培养液中轻易检测到的 NAD+，并支持流感嗜血杆菌在体外生长。相比之下，某些酿脓链球菌（A 组链球菌或 GAS）株通过分泌 NAD+-糖基水解酶（NADase）抑制流感嗜血杆菌在体外的生长，该酶降解细胞外 NAD+。相反，缺乏具有酶活性的 NADase 的 GAS 株释放细胞外 NAD+，这可以支持流感嗜血杆菌的生长。我们的结果表明，许多细菌物种，包括上呼吸道的正常菌群，都会将 NAD+释放到环境中。GAS 的独特之处在于它既能释放又能降解 NAD+。因此，流感嗜血杆菌与 GAS 共同定植在上呼吸道可能以菌株特异性的方式促进或限制其定植，这分别取决于 NAD+的释放或活性 NADase 的分泌。我们认为，除了作为宿主细胞的细胞毒素外，NADase 还可能通过限制流感嗜血杆菌在人类呼吸道中的生长来发挥单独的作用。
