BMS 114 · Common Core (Tronc commun)Section 5.4 · Bacterial growth, nutrition and culture
Section 21 of 33
Physical requirements for growth
Explain the chemical and physical conditions a bacterium needs before it can grow on an artificial medium, name an organism that illustrates each condition, and use those requirements to explain wound infection, food safety and laboratory culture.
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Why this matters
A bacterium does not grow merely because the medium is nutritious. It grows only when the whole environment suits it, and that environment is something a nurse can change. The oxygen inside a wound, the temperature of a refrigerator, the water left under a dressing, the salt in a preserved food and the light falling on a work surface all decide whether an organism multiplies or dies. Once the eight conditions are understood, the reasons behind seemingly arbitrary routines become visible: why an incubator is held at 35–37 °C, why a specimen must not sit in a warm room, why a deep dirty wound is a nursery for tetanus, and why salted fish keeps while fresh fish does not. These are the levers of infection control.
What you need to know first
You need only two ideas: that a bacterium is a living cell, and that bacteria are grown in the laboratory on a medium. Terms such as aerobe, anaerobe and enzyme are explained as they appear. The previous section described what a bacterium eats; this section describes the conditions under which it can eat and multiply.
Words to know (5)
Aerobe
A bacterium that can grow in the presence of oxygen; a strict or obligate aerobe cannot grow without it. Pseudomonas aeruginosa is a strict aerobe, so it grows best on the surface of a wound.
Obligate (strict) anaerobe
A bacterium that grows only when oxygen is absent and that is inhibited or killed by air. Clostridium tetani is found in dead tissue, never on a clean dry surface.
Capnophile
A bacterium that grows best in an atmosphere enriched with carbon dioxide, usually 5 to 10 %. Neisseria gonorrhoeae is a capnophile and is transported in a carbon dioxide-rich container.
Mesophile
A bacterium whose best growth occurs between about 25 °C and 40 °C, which includes most pathogens. Escherichia coli in a surgical wound grows well at the 37 °C of the human body.
Water activity (aw)
The fraction of the water in a food or medium that is free for microorganisms to use, on a scale from 0 to 1. Dried milk powder has a low water activity, so bacteria cannot grow in it even though it is nutritious.
One idea at a timeIdea 1 / 7
In one sentence Before a bacterium can grow on an artificial medium, eight conditions must be met: oxygen, carbon dioxide, pH, temperature, moisture, osmotic pressure, darkness and a sterile medium.
Eight conditions, not one
These eight requirements fall into two families. The chemical conditions — oxygen, carbon dioxide and pH — are properties of the molecules dissolved in the medium and around the cell, and they decide which metabolic reactions can run. The physical conditions — temperature, moisture, osmotic pressure and light — are properties of the environment that act on every cell at once: they set how fast enzymes work, whether water is available and whether ultraviolet light damages the genetic material. A sterile medium is a third kind of requirement, because it is not a stimulus to growth but the absence of competitors: the organism put into the medium must be the only one cultivated. Each condition has an optimum and a tolerable range, and a bacterium grows only when all eight fall inside its own range at the same time.
For example
A laboratory wants to grow a throat swab: it uses a buffered medium at pH 7.4, incubates the plate at 37 °C, keeps it moist in a sealed bag and sterilises every instrument and plate beforehand.
Pause and check yourself
A medium contains every nutrient a bacterium needs, yet nothing grows. Name three conditions outside nutrition that could be wrong.
Show the answer
Oxygen — the organism may be a strict anaerobe and the plate is sitting in air. Temperature — the incubator may be too cold or too hot for that organism. pH — the medium may be too acid or too alkaline. Water availability, osmotic pressure, light and contamination of the medium are further possibilities.
Read detailed explanations
Why oxygen tolerance is an enzymatic problem
Oxygen is the best terminal electron acceptor a cell can use, and also the source of its most dangerous by-products. When electrons travel down a respiratory chain, a small fraction escapes early and reduces oxygen incompletely, producing the superoxide radical and hydrogen peroxide. Both attack DNA, proteins and membrane lipids. The defence is enzymatic. Superoxide dismutase converts the superoxide radical into hydrogen peroxide and ordinary oxygen, and catalase or a peroxidase then converts the hydrogen peroxide into water. Whether an organism possesses a working set of these enzymes, and how much it possesses, decides where it can live far more precisely than a preference does. An obligate aerobe such as Pseudomonas aeruginosa has them in full. A facultative anaerobe such as Streptococcus pyogenes also has them, which is why it can respire when oxygen is present and ferment when it is not. A micro-aerophile such as Campylobacter jejuni has only low activity, so it survives the small amount of oxygen in a candle jar or a micro-aerophilic atmosphere but is damaged by the 21 % of air. An obligate anaerobe such as Clostridium lacks effective superoxide dismutase and catalase, so the same molecule that serves an aerobe as a fuel kills it. This is why a strict anaerobe dies in air but survives in a sealed container, and why the redox potential of the environment, not merely the presence of oxygen, matters. In clinical practice the consequence is direct: dead tissue from a crush injury, a deep puncture or a diabetic foot has a low redox potential and no blood supply, so it becomes an anaerobic incubator. Clostridium tetani spores germinate there and release tetanospasmin, and Clostridium perfringens can produce gas gangrene. Treatment therefore aims at the environment as much as at the organism: surgical debridement removes dead tissue, drainage and an open dressing restore oxygen, hyperbaric oxygen raises the tissue concentration of oxygen, and antibiotics such as metronidazole or benzylpenicillin are added. The laboratory reproduces the same logic with thioglycollate broth, an anaerobic jar and a gas-generating pack for strict anaerobes, and a candle jar for capnophiles and micro-aerophiles.
Temperature, moisture and osmotic pressure: the food-safety triangle
In food, the same three physical requirements act together, and preservation works by pushing at least one of them outside the range of the organisms present. Temperature is described by its cardinal points: below the minimum the membrane loses fluidity and transport stops; above the maximum enzymes denature and the cell dies; at the optimum growth is fastest. The familiar danger zone, roughly 5 °C to 60 °C, is simply the region in which the cardinal temperatures of most food-borne pathogens are all satisfied, so food left in it multiplies its bacterial load with every passing generation. Refrigeration does not kill: it slows metabolism, and psychrotrophic organisms such as Listeria monocytogenes and Yersinia enterocolitica continue to multiply slowly at 4 °C, which is why a refrigerated ready-to-eat food can still become dangerous. Freezing is even less lethal; it suspends growth, and the organisms resume when the food thaws. Moisture is better described by water activity than by dryness. Water activity, aw, is the fraction of the water in a food that is free for microorganisms to use, on a scale from 0 to 1, where pure water is 1.0. Most bacteria need an aw above about 0.90, Staphylococcus aureus can grow down to about 0.86, and halophilic moulds and yeasts go lower still. The old test of whether a food keeps is therefore not how wet it feels but how much of its water is free. Osmotic pressure acts through the same variable: adding salt or sugar binds water, lowers aw and draws water out of the bacterial cell by osmosis through its wall. That is why salted fish, cured meat and jam keep, and why the organisms that do spoil them are the halophiles and osmophiles, which tolerate high solute concentrations. Combining the three conditions — refrigeration plus salt or sugar plus drying — is called hurdle technology, because each hurdle that is too weak to stop growth alone may be sufficient in combination. Pasteurisation is the same idea applied with heat: it reduces the number of vegetative organisms but does not sterilise, because endospores and heat-resistant enzymes survive. For nursing, the rule is short: keep food out of the danger zone, remember that a refrigerator slows but does not kill, and treat a low water activity as a preservative rather than a guarantee.
Reproducing the eight conditions in the laboratory, and where nursing meets them
A requirement is useful only if it can be reproduced. In the culture room, each of the eight conditions corresponds to a piece of apparatus or a step. Temperature is controlled by an incubator held at 35–37 °C for pathogens and at 25 °C or 55 °C for psychrophiles and thermophiles, with the door opened as little as possible so that it recovers quickly. Carbon dioxide is supplied by a carbon dioxide incubator, or economically by a candle jar, in which a burning candle uses up oxygen and raises carbon dioxide; the same jar serves capnophiles and micro-aerophiles. Oxygen is removed for strict anaerobes inside an anaerobic jar with a gas-generating pack and a catalyst, or by using a freshly boiled, reducing medium such as thioglycollate broth, which also displays the growth pattern from the surface downwards. pH is held by a buffer and checked with a pH meter before sterilisation, because heating can shift it. Moisture is retained by sealing plates or by using a humidified incubator, since a plate that dries out will not support a demanding organism. Osmotic pressure is matched to the organism: a halophilic spoilage organism may need added salt, whereas a blood-containing medium is made isotonic to protect the red cells. Darkness matters when a medium contains a light-sensitive substance, and ultraviolet lamps are used to disinfect surfaces and safety cabinets rather than to incubate. Sterility is achieved by autoclaving at 121 °C and about 15 psi for 15 minutes, by filtration for heat-labile solutions, or by using pre-sterilised disposable plasticware, and it is maintained by aseptic technique at the bench. Nursing meets the same conditions in transport: a specimen that stays warm in a pocket overgrows, a specimen left in a hot vehicle may lose a delicate organism, a swab that dries out may kill Neisseria gonorrhoeae before the plate is inoculated, and a urine sample held at room temperature for hours changes its bacterial count. Knowing what each organism needs is therefore not laboratory trivia; it decides whether the result you receive is worth acting on.
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The eight conditions for growth, and the oxygen gradient that sorts bacteria
Read the top panel first: oxygen, carbon dioxide and pH are chemical conditions, while temperature, moisture, osmotic pressure and light are physical, and a sterile medium removes competitors. Then read the four tubes from top to bottom. The shading shows how oxygen falls with depth in a standing broth, and the dots show where each class of organism can actually grow: Pseudomonas at the surface, Campylobacter in a narrow band, Streptococcus throughout, Clostridium only at the bottom.
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01 / 6Chemical conditions: molecules in solution
If the drawing is wider than this panel, scroll it sideways. Labels stay readable; the numbered explanations below show the same ideas.
Chemical conditions: molecules in solution
Oxygen, carbon dioxide and pH are properties of the molecules dissolved in the medium and around the cell. They decide which metabolic reactions can run: oxygen is the terminal electron acceptor, carbon dioxide supplies the four-carbon acids of the Krebs cycle, and pH fixes the range in which enzymes keep their shape. Their optimum is narrow, so a buffered medium is used.
Schematic, not to scale: the tubes stand for the oxygen gradient in a column of broth, and the bands are drawn wider than they really are so that the growth pattern can be seen. The four organisms are typical examples of their oxygen class, not the only ones, and the enzyme rule underneath is a simplification of a family of defences against reactive oxygen species.
Key points to remember
✓Eight conditions decide growth: oxygen, carbon dioxide, pH, temperature, moisture, osmotic pressure, darkness and a sterile medium, grouped as chemical, physical and the sterility of the medium.
✓Oxygen is the final hydrogen acceptor; aerobes, strict aerobes, facultative anaerobes, micro-aerophiles and strict anaerobes are defined by whether they can use it and detoxify its reduction products.
✓All bacteria need a little carbon dioxide to replenish the Krebs cycle; capnophiles such as Neisseria gonorrhoeae need 5 to 10 % supplied in the atmosphere.
✓pH and temperature select the organisms that can grow: neutrophiles at pH 7.2–7.6, mesophiles at 25–40 °C, and incubators at 35–37 °C for pathogens.
✓Moisture, osmotic pressure and darkness are preservation tools as much as growth requirements: drying and salting stop growth without sterilising, and ultraviolet light is bactericidal.
Go deeper
Worked example
Reading a deep wound as a bacterial environment. A farmer arrives at the clinic with a deep, dirty laceration of the calf, received six hours ago and closed with a tight dressing. There is dead muscle at the base of the wound and no bleeding. You must explain to a colleague why this wound is dangerous, which of the eight growth requirements are already satisfied inside it, and what must be changed.
Read the wound as an environment, not only as an injury. A wound is a habitat with a temperature, a pH, a water content, a supply of nutrients and a concentration of oxygen. Dead muscle, dirt and clotted blood supply the nutrients; the tissue is at body temperature; it is wet; and no light reaches it. Six of the eight conditions are therefore met without any effort from the bacteria.
Identify the one condition that is missing, and ask which organism wants it missing. Torn vessels and dead muscle mean that oxygen no longer reaches the depths of the wound, so the redox potential falls. That is precisely the environment a strict anaerobe requires. Clostridium tetani spores, introduced with the soil, find here the anaerobic, moist, warm, dark, nutrient-rich niche in which they germinate.
Change the conditions rather than only the dressing. Surgical debridement removes the dead tissue, opens the wound to the air, restores blood supply and raises the oxygen concentration, so it attacks the anaerobic niche directly. A tight occlusive dressing that keeps the wound closed and airless does the opposite and should not be reapplied over dead tissue.
Add the treatment and the prevention that follow from the same reasoning. Tetanus is prevented by active immunisation with tetanus toxoid and, in a dirty wound, by human tetanus immunoglobulin; established infection is treated with metronidazole or benzylpenicillin together with adequate wound care. Antibiotics alone cannot correct an environment that keeps producing dead, oxygen-poor tissue.
The eight requirements turn a clinical picture into a set of controllable variables. Ask which conditions the site already provides, which one is decisive for the organism you fear, and which intervention changes that one. In this wound, oxygen is the decisive variable, and debridement — not a dressing — is the treatment that restores it.
Common misconception
Many beginners reduce bacterial growth to food and warmth, and treat the other six requirements as laboratory detail. That error leads directly to bad practice: a dressing kept tightly closed and moist because bacteria supposedly need warmth, a specimen left on a warm bench, or a preserved food assumed to be safe because it was refrigerated. A second and more dangerous error is to believe that a wound closed off from the air is protected. The opposite is true: closing a deep, dirty wound excludes oxygen, lowers the redox potential and creates exactly the anaerobic niche in which Clostridium species germinate.
Check your reasoning
Think of a patient you have seen with a wound, a urinary catheter or a food-related illness. Which of the eight conditions was the infection exploiting, and which single nursing action would have changed it?
Reveal an answer
Take a patient with a urinary catheter and a Proteus infection. The catheter provides a warm, moist, dark, nutrient-rich surface at body temperature, and the urine supplies the pH range the organism tolerates, so the decisive conditions are moisture and a surface that is supposed to be sterile but is no longer so. The single action that changes them is prompt removal of the catheter when it is no longer indicated, together with a closed drainage system and hand hygiene at every manipulation. Notice that the answer is environmental, not merely pharmaceutical.
Where this comes from
BMS 114 General Microbiology I lecture notes (supplied) · “Physical Conditions Required for Bacterial Growth”, lines 996–1092: the eight requirements, the oxygen groups, capnophiles and the Krebs cycle, the pH groups, the temperature groups, moisture and survival on drying, osmotic pressure, darkness and the sterile medium.
Correction to the supplied notes — the acid-and-alkali exception · Correction to the supplied notes: lines 1043–1044 state that the tubercle bacillus is the only exception to the lethality of 5 % hydrochloric acid or sodium hydroxide. Mycobacterium is notably more resistant than most bacteria because of its waxy, lipid-rich wall, but it is not the only exception; bacterial endospores and some other organisms also survive.
Correction to the supplied notes — water content and the cold virus · Correction to the supplied notes: line 1059 gives the water content as exactly 80 % of the living weight; the accurate figure is roughly 70–80 % and varies with species and growth phase. Lines 1068–1070 name “the common cold virus”; rhinovirus is the appropriate term, since many different viruses cause the common cold.
Correction to the supplied notes — the name of the gonococcus · Correction to the supplied notes: line 1029 writes “Neisseria gonorrhea”; the correct name is Neisseria gonorrhoeae. Section 5.6 also labels the same misspelling at line 1196.
Beyond the supplied notes — oxygen detoxifying enzymes, cardinal temperatures and water activity · superoxide dismutase, catalase and peroxidase as the determinants of oxygen tolerance; the cardinal temperatures (minimum, optimum, maximum) as the proper description of a temperature range; water activity as the measure of water available to microorganisms; and the survival of psychrotrophs such as Listeria monocytogenes and Yersinia enterocolitica in refrigeration.
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About this course and its sources
Bilingual lessons organised from the supplied BMS 114 General Microbiology I lecture notes (Mr Ngotelo F. Ambene, Mr Lyonga B. Lyonga & Mr Fumbui Louis Ngong). Diagrams are original educational schematics; sections that correct the notes or go beyond them say so in their sources. Nursing and French-language review are pending.