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All [https://en.wikipedia.org/wiki/Prokaryote prokaryotes], which includes all bacteria, are categorized based on the levels of oxygen in their environment in which they can grow and how they utilize oxygen, if at all <ref name="Todar_nutgro4"></ref>. ''Lactobacillus'' species are usually considered to be "facultative anaerobes" (or "facultative aerobes") <ref name="todar_lactics4"></ref>, however they are a special case. Facultative anaerobes usually make energy from oxygen if it is present via the [https://en.wikipedia.org/wiki/Oxidative_phosphorylation oxidative phosphorylation pathway], but otherwise engage in anaerobic fermentation <ref>[http://www.ncbi.nlm.nih.gov/books/NBK21208/ Biochemistry. 5th edition. Berg JM, Tymoczko JL, Stryer L. 2002. Chapter 18.]</ref><ref>[http://inst.bact.wisc.edu/inst/index.php?module=book&type=user&func=displayarticle&aid=111 Virtual Microbiology Textbook. Department of Bacteriology, University of Wisconsin-Madison. Retrieved 12/02/2015.]</ref>. ''Lactobacillus'' species can utilize oxygen, but not through the oxidative phosphorylation pathway. They use an alternative pathway instead. This pathway uses flavine-containing oxidases and peroxidases to carry out the oxidation of NADH2 using O2 <ref name="bergey">Bergey's Manual of Systematic Bacteriology, 2nd edition. pg 471</ref><ref>Correspondence with Bryan of Sui Generis Blog from Dan Pixley. 12/01/2015.</ref>. Lactobacilli, therefore, are unique in that they blur the line between facultative anaerobes and another class of prokaryotes known as "aerotolerant anaerobes". Aerotolerant anaerobes do not use oxygen to generate energy, but can grow in the presence of oxygen.

There are, however, a few exceptions to this in the scientific literature. For example, ''L. plantarum'', which is a facultatively heterofermentative species, is homolactic without the presence of oxygen and produces only lactic acid. After food sources have been exhausted and in the presence of oxygen, however, ''L. plantarum'' switches to heterolactic fermentation, and further converts lactic acid into acetic acid. In a lab setting the conversion of lactic acid to acetic acid only happened when glucose was no longer available (this may not be the case in wort where other limiting factors such as low pH can prevent ''L. plantarum'' from continuing their metabolic processes before all glucose is consumed, which is the case in kettle souring), and only during the stationary phase (after growth stopped). During the conversion of lactic acid to acetic acid, ''L. plantarum'' also produces hydrogen peroxide (H²0²), which is toxic to microorganisms and is thought to be a protection mechanism for ''L. plantarum'' <ref>[http://www.ncbi.nlm.nih.gov/pubmed/6480562 Physiological role of pyruvate oxidase in the aerobic metabolism of Lactobacillus plantarum. Sedewitz B, Schleifer KH, Götz F.1984.]</ref><ref name="Quatravaux_plantarum"></ref><ref name="microbewiki_plantarum">[https://microbewiki.kenyon.edu/index.php/Lactobacillus_plantarum_and_its_biological_implications Lactobacillus plantarum and its biological implications. Microbe Wiki. Retrieved 6/7/2015.]</ref><ref name="shaner_plantarum"></ref>. Omega Yeast Labs reports that no noticeable acetic acid is produced if the oxygen is not purged with their OYL-605 ''Lactobacillus'' blend which contains ''L. plantarum'', and brewers should not aerate wort during sour but purging O2 is not required <ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/1222363364458542/?comment_id=1222371257791086&reply_comment_id=1225212894173589&comment_tracking=%7B%22tn%22%3A%22R%22%7D MTF conversation with Adi Hastings from Omega Yeast Labs on the effects of oxygen presence in OYL-605. 02/02/2016.]</ref>. ''L. brevis'' has been shown to increase growth rates in the presence of oxygen <ref name="brevis_aeration"></ref>. Thomas Hübbe's master thesis showed that under lab growth media "VLB-S7-S", "NBB®-A", and MYPG + cyclohexmide, both strains of ''Lactobacillus'' tested (''L. brevis'' and ''L. parabrevis'') did not show growth in an aerobic chamber, but did grow in an anaerobic chamber <ref name="Hubbe">[https://www.facebook.com/groups/MilkTheFunk/1407620505932826/ Effect of mixed cultures on microbiological development in Berliner Weisse (master thesis). Thomas Hübbe. 2016.]</ref>.