If they do not, the term facultative is generally used. Very few organisms strictly require aerobic conditions for growth. So in most cases an organism is said to be either an anaerobe or a facultative organism. Sources such as the cerebrospinal fluid and blood should never harbor any bacterial organisms and therefore any and all organisms from these sources are considered dangerous and are indications for antimicrobial therapy. Clinical bacteria are classified according to how dangerous they are.
Pathogens are always likely to cause disease.
Bacteroides fragilis , or Clostridium difficile . Some organisms such as the Streptococcus viridans group can gain entry into the bloodstream and quietly become entrenched on the mitral valves of the heart causing a problem only after a long period as a cumulative effect. Some pathogens proactively create portals of entry. Perhaps a useful system of categorization would give an indication of the probability of infection by a particular organism. True, the CDC publishes a weekly report entitled MMWR and also publishes yearly figures on the incidence of infection of the various disease causing agents including bacteria which are thorough and authoritative.