Streptococcus pyogenes can support or inhibit growth of Haemophilus influenzae by supplying or restricting extracellular NAD⁺

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.