Jeon Experiment- Endosymbiotic Theory Evidence
7.1 Words and kinetics #
7.1.1 Four procedures and a D-value #
Sterilisation destroys or removes all viable life, including endospores and viruses. One cannot prove it by examining a sample and finding nothing; one runs a process whose chance of a survivor is made vanishingly small. Disinfection kills or inhibits pathogens on inert surfaces; spores often remain, and the agent is unfit for tissue. Antisepsis is the equivalent reduction on living tissue. Sanitisation lowers a load, often on food equipment, to a level a public-health standard accepts. Sanitised is not sterile. Bactericidal means killing; bacteriostatic means stopping growth so that something else must finish. Virucidal is a separate claim: wrecking an envelope may stop influenza and spare a naked picornavirus.
Killing under constant conditions is exponential. The D-value is the time, at a stated temperature, for a tenfold drop. If D is one minute, 10^6 becomes 10^5, then 10^4. That illustration is the definition, not a measurement on a named species. The z-value is the temperature rise that shortens D tenfold. A heavier load takes longer to reach the same probability of sterility. Low-acid canning is built around spores of C. botulinum and a reduction of the order of twelve decimal values. That is a probability statement, not a process schedule. A survivor is a cell with a generation time.
7.2 Physical and chemical agents #
7.2.1 Heat, filtration, and radiation #
Moist heat denatures protein more readily than dry heat at the same temperature. Boiling kills vegetative cells and many viruses, not endospores. An autoclave raises saturated steam, by pressure, to about 121 °C (about 15 lb in^−2 above atmospheric). Pressure is not itself the killing agent. The reference hold is about 15 minutes after the load is at temperature. Spores of Geobacillus stearothermophilus are the indicator because they outlast the pathogens of concern. Dry heat for glassware is traditionally about 160–170 °C for about two hours. Pasteurisation stops short of sterilisation: about 63 °C for 30 minutes, or about 72 °C for 15 seconds. Spores remain. Regimes above about 135 °C for a second or two, with aseptic packaging, are a different bargain. Repeated steaming catches spores only after they germinate. That is a remark about dormancy, not a procedure.
A membrane of about 0.2 µm pore size retains bacteria from a heat-labile fluid and does not reliably retain viruses. HEPA air filters catch particles around 0.3 µm, the most penetrating size, by impaction and diffusion as well as by sieving. Cold and low water activity preserve. They do not sterilise. Ultraviolet light near 260 nm makes pyrimidine dimers and does not penetrate dirt, glass, or shadow. Repair of those dimers is molecular biology. Ionising radiation breaks DNA through a packaged volume. No absorbed dose is stated. The difference that matters is penetration.
7.2.2 Chemical classes by mechanism #
Concentrations and mixing formulae are not part of this table. Organic matter protects cells and quenches oxidants.
| Class | Type members | Damage | Limit |
|---|---|---|---|
| Alcohols | aqueous ethanol or isopropanol, about 70% rather than absolute | denature protein, disrupt membranes; water is required | not sporicidal; poor in dirt |
| Halogens | chlorine, iodine, iodophors | oxidise and halogenate | chlorine dies in organic matter |
| Phenolics | phenol, cresols | membrane and protein; tolerate soil better than chlorine | not reliably sporicidal |
| Aldehydes | glutaraldehyde, formaldehyde | alkylate and cross-link | too toxic for skin |
| Ethylene oxide | the gas | alkylates protein and DNA; penetrates packs | specialist plant, then aeration |
| Heavy metals | silver, copper | bind enzyme thiols | slow; not general sterilants |
Lister’s phenol is the historical phenolic. Formaldehyde can turn an exotoxin into a toxoid if antigenicity survives loss of activity. The response to that antigen is immunology. Soaps remove organisms, which is the whole of the effect against alcohol-resistant spores such as those of C. difficile. Quaternary ammonium compounds disrupt membranes and prefer Gram-positive cells.
7.3 Antimicrobial drugs #
7.3.1 Targets, and static versus cidal #
Selective toxicity is the requirement. Doses and syntheses are omitted.
| Target | Examples | Spared in the host because |
|---|---|---|
| Peptidoglycan cross-link | beta-lactams; vancomycin on D-Ala–D-Ala | animals have no peptidoglycan |
| Mycolic acids | isoniazid | the pathway is acid-fast |
| 70S ribosome | aminoglycosides, tetracyclines (30S); macrolides, chloramphenicol (50S) | cytosolic ribosomes are 80S; the elongation step is molecular biology |
| Nucleic-acid enzymes | fluoroquinolones (gyrase); rifampicin (bacterial RNA polymerase) | the enzymes differ from the human ones |
| DNA after reductive activation | metronidazole, in anaerobes | aerobic host cells do not activate it the same way |
| Membrane or ergosterol | polymyxins; polyenes and azoles (fungi) | human membranes use cholesterol, not ergosterol |
| Folate synthesis | sulfonamides (mimic p-aminobenzoate); trimethoprim | humans absorb folate and skip the blocked step |
Beta-lactams kill only growing cells, because autolysins must be opening the wall. Aminoglycosides need a membrane potential for uptake and fail against many anaerobes. The minimum inhibitory concentration stops visible growth; the minimum bactericidal concentration kills. No assay is described. Narrow spectrum, as with penicillin G against many Gram-positive organisms and not against typical Gram-negative rods, leaves more of the microbiota alone. Broad spectrum selects widely and can open the way for C. difficile. Antifungal drugs use ergosterol or the fungal wall. Antiviral drugs use viral proteins from Chapter 5. A virus has neither peptidoglycan nor a 70S ribosome.
7.3.2 Resistance, spread, and stewardship #
Beta-lactamases hydrolyse the ring. Extended-spectrum enzymes and carbapenemases are the same idea against harder drugs. Clavulanic acid occupies some of the enzymes so a partner beta-lactam can still bind its target. PBP2a, from mecA, gives methicillin-resistant S. aureus a cross-linking enzyme with low beta-lactam affinity. Quinolone resistance alters gyrase. Macrolide resistance often methylates 23S rRNA. vanA replaces D-Ala–D-Ala with D-Ala–D-lactate, which vancomycin binds poorly. Efflux and porin loss keep drug out. A bypass enzyme ignores the drug. These are acquired one mutation at a time, or already assembled by horizontal transfer: conjugation of plasmids, transduction, transformation, and movement of cassettes by transposons and integrons. The machinery is genetics. The consequence is speed. Resistance reaches species that never met the drug.
Intrinsic resistance needs no new gene: an outer membrane, a missing wall, or anaerobic physiology that will not take up aminoglycosides. Stewardship follows. An antibacterial drug does not bind anything a virus offers. It does select resistance in the microbiota. Influenza and HIV are treated, when they are treated with drugs, at the viral enzymes already named. Clearance after a static drug, and immunity after a vaccine, are immunology.
7.4 Key terms, summary, and self-check #
Key terms. Sterilisation; disinfection; antisepsis; sanitisation; D-value; autoclave; pasteurisation; beta-lactamase; PBP2a; efflux; horizontal gene transfer; stewardship.
Summary. Only sterilisation includes spores. Killing is exponential, and D is one tenfold step. Autoclave pressure exists to raise steam above 100 °C; pasteurisation deliberately does not. Filtration removes cells that cannot be heated. Chemicals denature, oxidise, or alkylate. Drugs hit the wall, the 70S ribosome, gyrase or RNA polymerase, ergosterol, or folate synthesis. Resistance is destruction, a changed target, efflux, exclusion, or a bypass, and plasmids move it. An antibacterial drug has no viral target.
Self-check.
1. Which procedure destroys endospores, and why is sanitised not sterile?
2. D is 1.5 minutes. How long takes 10^6 cells to 10^2? Why is the time useless at a lower temperature?
3. Why is autoclave pressure used, and why does 72 °C for 15 seconds not replace it?
4. Name the targets of a beta-lactam, a fluoroquinolone, a polyene, and a sulfonamide. Why does the polyene miss E. coli?
5. What substrate change defeats vancomycin, and why does penicillin not treat influenza?
—
—
—