Physical methods of microbial control will typically sterilize, whereas chemical methods will only disinfect.

A. All of these choices are correct: Effective antimicrobial drugs must meet several important pharmacologic and microbiologic criteria to ensure therapeutic success. These include selective toxicity toward microbial cells, minimal harmful effects on human tissues, the ability to remain active in body fluids and tissues, and the capacity to reach the site of infection in effective concentrations. Each listed characteristic contributes to the effectiveness and safety of therapy.

B. do not cause serious side effects in humans: One of the essential principles of antimicrobial therapy is selective toxicity, meaning the drug targets microbial structures or metabolic pathways while minimizing harm to host cells. Antimicrobials should produce manageable adverse effects so that treatment can proceed without causing significant injury to the patient.

C. remains active in body tissues and fluids: For an antimicrobial to be effective, it must maintain adequate stability and activity within physiological environments such as blood, tissues, and extracellular fluids. Drugs that remain stable and active in the body are better able to sustain antimicrobial activity against pathogens.

D. readily delivered to the site of infection: Antimicrobial agents must be capable of reaching the infected tissues in sufficient concentrations to inhibit or destroy the pathogen. Barriers such as poor blood supply and tissue inflammation can limit drug delivery. Effective drugs are able to penetrate these tissues or are administered in ways that ensure adequate distribution.

E. high toxicity against microbial cells: Antimicrobial drugs should exert strong inhibitory or lethal effects on microorganisms by targeting structures such as bacterial cell walls, ribosomes, nucleic acids, or metabolic pathways. Selective toxicity allows pathogens to be eliminated while host cells remain largely unaffected. This is characteristic of effective antimicrobial therapy.

A. saprobic: A saprobic relationship refers to organisms that obtain nutrients by decomposing dead or decaying organic matter. Saprobes, such as many fungi and bacteria, play an important role in environmental nutrient cycling. The E. coli in the human intestine are not primarily decomposing dead tissue but instead living within a host and interacting with host physiology.

B. commensal: In a commensal relationship, one organism benefits while the other is neither significantly helped nor harmed. Although many normal microbiota are considered commensal, intestinal E. coli provide a measurable benefit to the host by synthesizing vitamin K and certain B vitamins. The host receives a physiological benefit, this interaction is more than commensal.

C. parasitic: A parasitic relationship occurs when one organism benefits at the expense of the host, often causing harm or disease. Pathogenic bacteria demonstrate parasitism by damaging tissues, stealing nutrients, or producing toxins. E. coli that reside in the colon as part of the normal microbiota do not harm the host and instead contribute to normal metabolic functions.

D. mutualistic: Mutualism describes a symbiotic relationship in which both organisms benefit. In the human colon, E. coli receive nutrients and a stable environment for growth, while the host benefits from bacterial synthesis of vitamin K, which is essential for blood clotting processes. This reciprocal exchange of benefits characterizes a mutualistic relationship.