The One Idea Everything Else Hangs on
Start with a single sentence and build outward from it: an antibiotic works only because the bacterium is built differently from your patient's cells. Every term in this chapter is a consequence of that one fact. The drug must find a structure or pathway the microbe has and the human host does not, reach it at the infected site, and act before the microbe neutralises it. When all three hold, the drug helps. When any one fails, the drug is the wrong choice, no matter how the laboratory report reads.
This difference has a name: selective toxicity — harming the microbe far more than the host. A bacterium has a peptidoglycan cell wall, a 70S ribosome, its own folate-synthesis machinery and a distinct DNA-handling apparatus; human cells have none of the first, an 80S ribosome, preformed dietary folate and different replication enzymes. Each of those differences is a potential drug target, and the size of the gap between microbial effect and host harm sets how usable a drug is. Polymyxins act on a cell membrane that is too similar to ours, so they are toxic and used sparingly; β-lactams act on wall-building enzymes we simply do not have, so they are remarkably safe. Hold this idea — bug-minus-host difference — and the rest of the chapter is just naming the differences, the language clinicians use to describe them, and the ways microbes erode the gap.
From there the build is deliberate: first what a bacterium is (structure), then why that structure creates drug targets and defines spectrum, then how the laboratory measures and reports activity (MIC, breakpoints, susceptibility), then how the drug behaves in the body to reach the target (pharmacokinetics and pharmacodynamics), then how microbes fight back (resistance), and finally the specimen and stewardship logic that ties the science to a real woman on the ward. Each section assumes only what came before it.
Antibiotic Language in O&G
With selective toxicity as the anchor, the clinical vocabulary becomes a set of decisions about which difference you are exploiting and when. The same woman may need prophylaxis before a predictable inoculation event, empiric therapy before cultures are known, targeted therapy after susceptibility results, or no antibiotic at all when a swab reports colonising flora. These choices follow from microbial burden, site of infection, likely organisms, host immunity, drug exposure and resistance selection.
Primary level: define the terms and explain why they matter. Intermediate and Final then apply them to treatment choices in Intermediateacute pelvic infection, IntermediateSTI pathology, Finalpelvic inflammatory disease, and Finalmaternal sepsis.
Core Definitions
| Term | Rigorous meaning | O&G application |
|---|---|---|
| Antimicrobial | Any agent active against microbes, including bacteria, viruses, fungi and parasites. | "Antimicrobial" is broader than "antibiotic"; it includes aciclovir, fluconazole and metronidazole as well as antibacterial drugs. |
| Antibiotic / antibacterial | A medicine used to inhibit or kill bacteria. In clinical use, "antibiotic" usually means antibacterial. | Beta-lactams, aminoglycosides, macrolides, tetracyclines and glycopeptides. |
| Antiseptic | A chemical used on living tissue to reduce microbial load. | Chlorhexidine skin preparation before caesarean section. |
| Disinfectant | A chemical used on inanimate objects or surfaces to reduce pathogens. | Theatre surface disinfection or probe reprocessing; see Intermediatesurgical instruments and safe use. |
| Empiric therapy | Treatment started before the pathogen and susceptibility are known, based on syndrome, severity, local epidemiology and patient risk factors. | Early cover for a likely infection while culture and source information are pending. |
| Targeted therapy | Treatment chosen after the organism and susceptibility pattern are known. | Narrowing therapy when a pathogen and its susceptibility pattern explain the syndrome. |
| Prophylaxis | Antibiotic given to prevent infection around a predictable exposure, not to treat established infection. | A prevention concept, distinct from treating established pelvic, urinary or wound infection. |
| Pre-emptive therapy | Treatment given because a high-risk exposure or early marker predicts disease before full clinical infection. | Used in selected specialist contexts; the Primary point is that it differs from empiric treatment. |
| Suppressive therapy | Long-term treatment to reduce recurrence or maintain control when eradication is difficult. | Recurrent urinary infection in pregnancy is a clinical decision; the Primary concept is ongoing selection pressure. |
| Source control | Removal or drainage of the focus that antibiotics cannot reliably sterilise. | Evacuation of retained infected products, drainage of abscess, removal of infected foreign material. |
The exam trap is to treat "antibiotic" as a complete answer. In O&G, the correct answer often begins earlier: Is there infection, colonisation, contamination or inflammation? Is the focus accessible to drugs? Are cultures needed first? Is there an infected collection that requires drainage?
Three further terms sit alongside antiseptic and disinfectant and are often confused, because they describe how completely microbes are removed rather than which drug is used. Decontamination is the general reduction of microbial load to a safe level. Disinfection kills actively dividing organisms but does not reliably destroy bacterial spores or all viruses, so it lowers the count substantially without guaranteeing zero. Sterilisation is the absolute end of that scale — the removal of all microorganisms including spores, defined as such a low probability of a surviving organism that a single survivor in a million processed items would count as failure. The practical point for O&G is the order: disinfection (including cleaning) precedes sterilisation, because organic debris and a high starting bioburden defeat the sterilising step. The instrument-processing detail belongs to Intermediatesurgical instruments and safe use; the Primary concept is only that these words name different end-points, not interchangeable synonyms.
What a Bacterium Is, and Why It Has Targets
Because selective toxicity depends on the differences between microbe and host, the next step is to look at how a bacterium is built. A bacterial cell is a single compartment bounded by a cell membrane, wrapped in a rigid cell wall whose load-bearing polymer is peptidoglycan — a mesh of sugar chains cross-linked by short peptides. The enzymes that forge those cross-links are the penicillin-binding proteins (PBPs). Human cells have no wall and no PBPs at all, which is exactly why a drug that jams those enzymes can kill the bacterium and barely touch the patient.