10.3 HIV, Antibiotics and Resistance

Human immunodeficiency virus is an enveloped retrovirus whose genome is RNA. Viral surface proteins enable entry into susceptible immune cells, especially helper T cells bearing appropriate receptors. Reverse transcriptase makes a DNA copy of viral RNA, and viral DNA can integrate into host-cell DNA. The integrated form explains persistence: a cell can retain viral information during a clinically quiet period and later produce new virus.

HIV infection and AIDS are not synonyms. A person may carry HIV for years without the collection of illnesses defining acquired immune deficiency syndrome. As helper T-cell numbers and function decline, activation of B cells, cytotoxic T cells and macrophages becomes less effective. Opportunistic infections and some cancers then occur because immune coordination has failed. AIDS is the advanced clinical consequence of untreated or unsuccessfully controlled HIV infection.

HIV can be transmitted when infected blood, semen, vaginal fluids or breast milk gain access to another person's tissues, and transmission can occur during pregnancy or birth. It is not spread by ordinary skin contact, shared air or casual social interaction. Screening donated blood, sterile injection equipment, barrier methods and antiretroviral treatment reduce transmission. Effective therapy suppresses replication but does not simply excise every integrated viral sequence.

The consequences reveal why helper T cells matter. A direct count of pathogen-killing cells gives an incomplete picture of immune capacity: signalling and clonal activation are system-level functions. Removing a coordinating population can weaken both humoral and cell-mediated branches even if many B cells and cytotoxic T cells remain physically present.

Combination antiretroviral therapy targets different stages of HIV replication, reducing the probability that one resistant viral variant can escape every drug. Suppressing viral load protects helper T-cell function and greatly lowers transmission risk. The logic resembles combination control in other evolving populations: simultaneous independent barriers are harder to evade than one barrier, although adherence, access, toxicity and resistance monitoring remain important.

Antibiotics are substances that kill bacteria or inhibit their growth at concentrations tolerated by the host. Selective toxicity is possible because bacterial cells differ from human cells. Some antibiotics interfere with peptidoglycan wall synthesis; others bind bacterial ribosomes and inhibit translation; further classes disrupt bacterial enzymes or nucleic-acid processes. A useful drug target must be important to the bacterium and sufficiently different from the corresponding host process.

Antibiotics do not treat viral infections. A virus lacks a peptidoglycan wall and bacterial ribosome and depends on host-cell machinery for much of its replication. An antibiotic aimed at bacterial wall synthesis therefore has no target in an influenza virion or infected human cell. Antiviral drugs require other targets, such as virus-specific enzymes or entry steps, and must avoid intolerable damage to host processes.

A bactericidal antibiotic kills susceptible bacteria, whereas a bacteriostatic antibiotic stops or slows their reproduction so host defences can clear them. The distinction depends partly on concentration and organism. Neither label implies effectiveness against every bacterium: cell envelope, drug uptake, target sequence and metabolic state all influence susceptibility, so laboratory identification and sensitivity testing can guide treatment.

A bacterial population may already contain resistant variants before treatment because mutation and gene transfer generate variation. Antibiotic exposure does not make each bacterium adapt purposefully. Susceptible cells die or stop dividing, while resistant cells leave more descendants. The frequency of resistance alleles therefore rises. Treatment is the selecting condition; it is not necessarily the original source of the resistant allele.

Resistance can arise through altered drug targets, enzymes that inactivate the drug, reduced entry, increased efflux or bypass pathways. Plasmids can carry resistance genes between bacteria, including between strains or species, by horizontal gene transfer. A plasmid carrying several resistance genes can create multidrug resistance when selection for one gene preserves the whole plasmid.

Every unnecessary exposure creates an opportunity for susceptible competitors to be removed. Overprescribing, using antibiotics for viral illness, incorrect dosing, poor infection control and widespread agricultural use can all increase selection or transmission. Completing a prescribed course should follow current clinical guidance: the biological aim is adequate pathogen clearance without avoidable exposure, and individual patients should not invent dosing rules from a general evolutionary principle.

Many antimicrobial molecules originate in ecological competition among microorganisms. Penicillin was observed because a Penicillium mould inhibited nearby bacteria, but observation alone did not produce a medicine: isolation, purification, dose testing, safety studies and scalable production were required. Modern searches culture previously inaccessible soil organisms, examine marine organisms and screen large chemical libraries. Computational methods can prioritize molecules with structural features associated with antibacterial activity, but predictions still require experimental testing.

A clear zone around a substance on a bacterial lawn is evidence of inhibition under those conditions, not proof that the substance is safe or effective in a patient. Researchers must determine spectrum, minimum inhibitory concentration, toxicity, stability, distribution and the likelihood of resistance. Gram-negative bacteria can be especially difficult targets because an outer membrane restricts entry of many molecules. Drug discovery therefore connects cell structure, evolution and evidence quality.