Physical and Chemical Control of Microbes
Antibiotics treat infection after microbes reach tissue. Sterilisation, disinfection, antisepsis and pasteurisation prevent infection by reducing microbial transfer before invasion occurs. This matters wherever instruments, skin, mucosa, surfaces, milk or specimens can carry organisms into an O&G patient or newborn.
Primary level: define the control methods and explain why they work. Intermediate and Final apply them to theatre practice, device policies, perioperative care and infected-patient escalation in Intermediatesurgical instruments and safe use, IntermediateERAS principles, and Finalmaternal sepsis.
The exam logic is:
what is the item -> what tissue will it touch -> what organisms might be present -> what level of microbial kill is required -> how do we prove the process worked?
This prevents the common error of choosing a process because it sounds strong rather than because it matches the clinical risk.
The One Idea Everything Rests on: Killing Is Probability, Not Magic
Before any classification or method makes sense, anchor on a single counter-intuitive fact: microbial killing is a statistical process, not a switch. When you apply heat or a chemical to a population of organisms, you do not wipe them out instantly. Instead, a roughly constant proportion of the survivors dies in each equal slice of time, so the number alive falls in a predictable curve rather than dropping to zero in one step.
The practical shorthand for this is the decimal-reduction time, written D-value: the time a given process needs to kill 90% of the population, that is, to reduce it by one log₁₀ (a factor of ten). After one D-value, one in ten survives; after two, one in a hundred; after six, one in a million. Two consequences flow from this single idea, and they explain almost everything that follows:
- More organisms at the start means more time (or harsher conditions) to reach safety. A process that easily handles a light contamination can fail on an instrument caked with blood and tissue, because you are starting many logs higher up the curve. This is why cleaning is the first killing step, not a cosmetic preliminary.
- You can never prove zero — only a very small probability. Because the curve approaches zero without ever mathematically reaching it, "sterile" is defined as a probability of a surviving organism, not a guarantee of none. That probability is the sterility assurance level (SAL), and the recognised target for items entering sterile tissue is a one-in-a-million chance (10⁻⁶) of a single viable organism surviving on a processed item.
Hold this model in mind. Sterilisation, disinfection, antisepsis and pasteurisation are simply different points on the same survival curve — chosen to match how dangerous the contact is and how resistant the likely organisms are.
The Microbial Control Ladder
Microbial control is a ladder, not a single action. Each step has a different target.
| Level | Definition | What survives | O&G example |
|---|---|---|---|
| Cleaning | Physical removal of soil, blood, mucus, biofilm and many microbes using water, detergent and mechanical action. | Some microbes remain, but bioburden falls. | Removing blood from instruments before reprocessing. |
| Decontamination | Making an item safer to handle by reducing contamination. | Depends on method. | Point-of-use handling of soiled instruments. |
| Low-level disinfection | Kills many vegetative bacteria and enveloped viruses on inanimate objects. | Mycobacteria, spores and some non-enveloped viruses may survive. | Non-critical environmental surfaces if not visibly contaminated with high-risk material. |
| Intermediate-level disinfection | Kills vegetative bacteria, most viruses and mycobacteria. | Spores may survive. | Some surface or equipment decontamination where tuberculosis-level activity matters. |
| High-level disinfection | Kills all microorganisms except high numbers of bacterial spores. | Heavy spore contamination can survive. | Reprocessing selected semi-critical equipment when sterilisation is not possible. |
| Sterilisation | Destroys all forms of microbial life, including bacterial spores. | No viable microbes should remain if process is valid. | Reusable surgical instruments entering sterile tissue. |
| Antisepsis | Chemical reduction of microbes on living tissue. | Skin is never made sterile. | Surgical hand preparation and skin preparation before incision. |
| Asepsis | Practices that prevent contamination of sterile tissue or equipment. | Not a killing method; it prevents transfer. | Sterile draping, glove technique, closed instrument handling. |
| Pasteurisation | Controlled heat process that reduces pathogens in liquids or foods without making the product sterile. | Spores and some heat-tolerant organisms may survive. | Donor human milk processing and avoidance of unpasteurised foods in pregnancy. |
Bioburden is the number and type of microbes present before processing. A method that reliably kills a small inoculum may fail when instruments are covered with blood, tissue or mucus. Cleaning is therefore not cosmetic; it is the first microbiological killing step because it removes the shield that protects organisms.
Control Method Versus Clinical Aim
| Clinical aim | Correct concept |
|---|---|
| Make a surgical instrument safe to enter the uterus/abdomen | cleaning followed by validated sterilisation |
| Prepare skin before incision | antisepsis, not sterilisation |
| Clean a CTG belt after intact-skin contact | cleaning and disinfection according to contamination risk |
| Reprocess a transvaginal probe | cleaning, cover use and validated high-level disinfection/sterilisation according to device policy |
| Reduce pathogen transmission in donor milk | pasteurisation plus cold-chain and handling control |
| Reduce theatre wound infection | asepsis plus sterilised instruments plus prophylaxis plus tissue care |
What Makes Microbes Hard to Kill?
Resistance to physical and chemical control is not the same as antibiotic resistance. It depends on structure, metabolic state and protection from contact.
| Microbial form | Why it resists control | Practical implication |
|---|---|---|
| Bacterial spores | Dehydrated, dormant, thick protective coat. | Require sterilisation-level processes; ordinary disinfection is insufficient. |
| Mycobacteria | Lipid-rich cell wall resists many chemicals. | Important for high-level disinfection standards and TB-aware infection prevention. |
| Non-enveloped viruses | Lack lipid envelope, often environmentally stable. | Alcohol and low-level agents may be less reliable than for enveloped viruses. |
| Enveloped viruses | Lipid envelope is vulnerable to detergents and alcohols. | HIV and many respiratory viruses are relatively susceptible outside the host, though transmission precautions remain essential. |
| Biofilm organisms | Matrix blocks penetration and organisms grow slowly. | Devices and surfaces require mechanical cleaning; chemicals alone may not reach embedded bacteria. |
| Prions | Misfolded proteins, not living organisms. | Require special policies; routine sterilisation assumptions may not apply. |
Organic matter reduces activity by chemically inactivating disinfectants, blocking contact and creating microenvironments. Contact time, concentration, temperature, pH, water quality and correct storage all matter. A disinfectant label is not magic; it assumes the item was cleaned and the product was used at the correct dilution for the correct duration.