How would you classify organisms that live together in close association, with benefits that may vary?

Organisms that live together in close association, where the benefits can vary for each organism involved, are best classified as commensals. In a commensal relationship, one organism benefits while the other is neither helped nor harmed. This type of association is common in microbial ecosystems, where the presence of one species can provide advantages, such as access to nutrients or enhanced survival, to another without causing any detriment to it.

In contrast, pathogens are organisms that cause disease in their host, while parasites derive benefits at the expense of their host. Opportunistic pathogens are usually harmless but can exploit situations such as a weakened immune system to cause disease, indicating a relationship that leans towards harmful rather than neutral. Thus, the defining characteristic of commensals is their ability to coexist with another organism in a way that can be beneficial to one without affecting the other negatively, making this classification the most accurate in the context provided.