Broad-spectrum antibiotic estimated 10-100 trillion multiple organisms that colonize the human body. As a side-effect of therapy, antibiotics can change the body's normal microbial content by attacking indiscriminately both the pathological and naturally occurring, beneficial or harmless bacteria found in the intestines, lungs and bladder. The destruction of the body's normal bacterial flora is thought to disrupt immunity, nutrition, and lead to a relative overgrowth in some bacteria or fungi. An overgrowth of drug-resistant microorganisms can lead to a secondary infection such as "Clostridium difficile" ("C. diff") or candidiasis ("thrush"). This side-effect is more likely with the use of broad-spectrum antibiotics, given
Bacteria a molecular signal is called quorum sensing, which serves the purpose of determining whether there is a local population density that is sufficiently high that it is productive to invest in processes that are only successful if large numbers of similar organisms behave similarly, as in excreting digestive enzymes or emitting light. Quorum sensing allows bacteria to coordinate gene expression, and enables them to produce, release and detect autoinducers or pheromones which accumulate with the growth in cell population. Classification seeks to describe the diversity of bacterial species by naming and grouping organisms based on similarities. Bacteria can be classified