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Background  Aerobically-grown bacteria can be challenged by hydrogen peroxide stress from endogenous aerobic metabolism and exogenously generated reactive oxygen species. Catalase (Kat), alkyl hydroperoxidase (Ahp), and glutathione peroxidase (Gpx) systems are major adaptive responses to H 2 O 2  stress in bacteria.  Stenotrophomonas maltophilia  is a ubiquitous Gram-negative bacterium equipped with four Kats (KatA1, KatA2, KatMn, and KatE), one Ahp (AhpCF), and three Gpxs (Gpx1, Gpx2, and Gpx3). Here, we systematically investigated how the eight H 2 O 2  scavenging genes differentially contribute to the low-micromolar levels of H 2 O 2  generated from aerobic metabolism and high-millimolar levels of H 2 O 2  from exogenous sources.    Methods  Gene expression was assessed and quantified by reverse transcription-PCR (RT-PCR) and real time quantitative PCR (qRT-PCR), respectively. The contribution of these enzymes to H 2 O 2  stress was assessed using mutant construction and functional investigation.    Results  Of the eight genes,  katA2 ,  ahpCF , and  gpx3  were intrinsically expressed in response to low-micromolar levels of H 2 O 2  from aerobic metabolism, and the expression of  katA2  and  ahpCF  was regulated by OxyR. AhpCF and KatA2 were responsible for alleviating aerobic growth-mediated low concentration H 2 O 2  stress and AhpCF played a critical role for stationary-phase cells.  KatA2  was upregulated to compensate for AhpCF in the case of  ahpCF  inactivation. After exposure to millimolar levels of H 2 O 2 ,  katA2  and  ahpCF  were upregulated in an OxyR-dependent manner. KatA2 was the critical enzyme for dealing with high concentration H 2 O 2 . Loss-of-function of KatA2 increased bacterial susceptibility to high concentration H 2 O 2 .    Conclusions  AhpCF and KatA2 are key enzymes protecting  S. maltophilia  from hydrogen peroxide stress.

Protection from hydrogen peroxide stress relies mainly on AhpCF and KatA2 in Stenotrophomonas maltophilia