Some of the obligate and facultative anaerobic bacteria are associated with BV. Intracellular parasites can be either facultative or obligate in nature. Fish can be obligate or facultative shoalers. The least common were infections caused exclusively by anaerobes and facultative organisms. Some Bacillaceae are aerobic, while others are facultative or strict anaerobes. Air breathing fish can be divided into obligate air breathers and facultative air breathers. This association may be either facultative or obligate. CHG is active against Gram-positive and Gram-negative organisms, facultative anaerobes, aerobes, and yeasts. There are organisms that can adjust their metabolism to their environment, such as facultative anaerobes. Plants that flower in response to photoperiod may have a facultative or obligate response. It shows consistent growth throughout nutrient broth which is consistent with being a facultative anaerobe. It is a beta-hemolytic, catalase-negative, and facultative anaerobe. Monogamy can come in two forms; obligate and facultative. An alternative to contrasting nature and nurture focuses on "obligate vs. facultative" adaptations. Citrobacter freundii is a species of facultative anaerobic gram-negative bacteria of the family Enterobacteriaceae.