Overview
Bacteria are prokaryotic organisms that lack a nuclear membrane, nucleolus and cell organelles such as mitochondria and the golgi apparatus. Understanding how they obtain nutrients, the gases and temperatures they require, and how they multiply is fundamental to every technique used in the clinical microbiology laboratory — from culture media selection to incubation conditions.
This lesson explains bacterial nutritional classification, gaseous and temperature requirements, binary fission, and the four phases of the bacterial growth curve — knowledge that directly determines how organisms are cultured, identified and counted in practice.
Learning Objectives
After this lesson you will be able to…- Discuss the nutritional, gaseous and temperature requirements for bacterial growth
- Classify bacteria based on their energy source and ability to synthesise metabolites
- Describe how bacteria grow and multiply by binary fission
- List the salient features of the bacterial growth curve
- Differentiate total count from viable count methods for bacterial enumeration
Clinical Story
Why This MattersA urine sample is sent for culture from a patient with suspected urinary tract infection. The lab technologist knows that Escherichia coli, a facultative anaerobe with a generation time of about 20 minutes, will produce countable colonies on blood agar within 18–24 hours at 37°C — but if the specimen is left at room temperature overnight before plating, contaminating organisms will multiply exponentially and give a falsely high or misleading colony count. Understanding bacterial growth kinetics is what separates an accurate culture report from a misleading one.
Core Concepts
The bacterial cell contains water (80% of total weight), proteins, polysaccharides, lipids, nucleic acids, mucopeptides and low molecular weight compounds. Minimum nutritional requirements are water, a carbon source, a nitrogen source, and inorganic salts (anions such as phosphate and sulphate; cations such as sodium, potassium, magnesium, iron and calcium).
Bacteria are classified by energy source: phototrophs derive energy from sunlight; chemotrophs derive it from chemical reactions. By synthetic ability: autotrophs synthesise all their own organic compounds from CO₂ and nitrogen; heterotrophs depend on preformed organic compounds and have widely varying requirements. Some bacteria need trace organic compounds called growth factors or bacterial vitamins (e.g. thiamine, riboflavin, folic acid, B12).
Obligate aerobes grow only in the presence of oxygen (e.g. cholera bacillus). Facultative anaerobes are ordinarily aerobic but can grow without oxygen, though less abundantly — most medically important bacteria fall here. Obligate anaerobes (e.g. Clostridia) grow only in the absence of oxygen and may even die on exposure to it. Microaerophilic bacteria grow best with low oxygen tension.
Aerobes use oxidative phosphorylation (oxygen as final electron acceptor, converting ADP to ATP). Anaerobes commonly use fermentation — breaking down glucose without oxygen via substrate-level phosphorylation, producing acids, alcohols and gas. Capnophilic bacteria like Brucella abortus require higher CO₂ levels (5–10%) for growth.
Each species has a temperature range with an optimum temperature — 37°C for most pathogenic bacteria. Mesophilic bacteria grow best at 25–40°C (e.g. E. coli). Psychrophilic bacteria grow best below 20°C (soil/water saprophytes, cause of refrigerated food spoilage). Thermophilic bacteria grow best at 55–80°C and may spoil under-processed canned food; some, like Geobacillus stearothermophilus, form highly heat-resistant spores.
Bacteria divide by binary fission. The time for one bacterium to become two daughter cells is the generation time — about 20 minutes for E. coli, 20 hours for tubercle bacilli, and 20 days for lepra bacilli.
When plotted over time, bacterial counts trace the growth curve: Lag phase (adaptation, no appreciable increase in number); Log phase (exponential multiplication, cells smaller, stain uniformly); Stationary phase (nutrient depletion and toxic buildup halt net growth, cells frequently Gram-variable, sporulation occurs); Phase of decline (cell death exceeds new cell formation due to nutrient exhaustion, toxin accumulation and autolytic enzymes).
Laboratory Principle
Viable bacterial counting relies on the principle that a single living, dividing cell will multiply on solid media to form a visible colony — so the number of colonies formed after incubation is a direct estimate of the number of viable organisms originally present in the sample. In the dilution method, a suspension is serially diluted until unit quantities no longer reliably yield growth, and the viable count is statistically derived from the pattern of positive and negative tubes — the principle behind the presumptive coliform count used to assess drinking water contamination.
Equipment Required
Reagents & Materials
| Reagent / Material | Concentration / Grade | Purpose | Storage |
|---|---|---|---|
| Nutrient broth | Standard formulation | Liquid medium for viable dilution counts | 2–8°C, protect from contamination |
| Blood agar | 5% sheep blood base | General purpose solid culture medium | 2–8°C, use before expiry |
| MacConkey agar | Selective/differential | Isolates and differentiates Gram-negative bacilli | 2–8°C |
| Peptone water diluent | 0.1% sterile | Serial dilution of suspensions for counting | Room temperature, sterile |
| Sterile saline | 0.85% NaCl | Suspension of organisms for counting/inoculation | Room temperature, sterile |
Step-by-Step Procedure — Viable Count by Plating (Miles & Misra Method)
Make ten-fold serial dilutions of the bacterial suspension in sterile peptone water or saline, using fresh pipette tips for each dilution to avoid carry-over.
Ensure agar plates are surface-dried before use so that the drops of diluted suspension are absorbed rather than running together.
Place measured drops of each dilution onto marked, labelled sections of the dried agar surface.
Allow drops to be absorbed, then incubate the plate at the organism's optimum temperature (commonly 37°C) for 18–24 hours.
Count the colonies in the dilution that gives a countable number (typically 30–300), and back-calculate the viable count per millilitre of the original suspension using the dilution factor.
Flow Diagram — Bacterial Growth Curve
Quality Control
Use standard reference strains with known generation times (e.g. ATCC E. coli) to periodically verify that incubator temperature, media performance and dilution technique give the expected colony counts. Include an uninoculated (sterility) control plate with each new batch of media.
Participate in external proficiency testing panels for quantitative bacteriology (e.g. water testing EQA schemes) that periodically distribute blinded samples for viable count comparison across participating laboratories.
Reference Values
Typical Generation Times & Growth Conditions⚠️ Reference ranges may vary between laboratories. Always apply your laboratory's established reference intervals.
Clinical Interpretation
| Finding | Possible Significance | Action / Follow-up |
|---|---|---|
| High colony count on urine culture (>10⁵ CFU/mL) | Suggestive of significant bacteriuria / urinary tract infection | Identify organism, perform antibiotic sensitivity testing |
| No growth after 24–48 hours incubation | May indicate a fastidious, anaerobic, or capnophilic organism needing special conditions, or a true negative culture | Extend incubation, use enriched media or CO₂/anaerobic conditions as indicated |
| Delayed/no growth of suspected TB culture at standard incubation time | Consistent with very long generation time of M. tuberculosis (~20 hours) | Extend incubation up to 6–8 weeks on suitable media (e.g. Lowenstein-Jensen) |
Common Errors & How to Avoid Them
Cause: Leaving a specimen at room temperature allows contaminating or commensal organisms to multiply exponentially during the log phase, giving falsely elevated colony counts.
Prevention: Process specimens promptly or refrigerate at 2–8°C if delay is unavoidable.
Cause: Incubating outside an organism's optimum temperature range slows or prevents growth, leading to false-negative cultures.
Prevention: Verify and log incubator temperature daily; use 37°C for most pathogens unless a specific organism requires otherwise.
Cause: Plates with too few or too many colonies (outside ~30–300) give statistically unreliable viable counts.
Prevention: Use appropriate serial dilutions so at least one plate falls within the countable range.
Cause: Culturing an obligate anaerobe under aerobic conditions (or vice versa) results in no growth, mimicking a false-negative culture.
Prevention: Select the correct incubation atmosphere (aerobic, anaerobic, microaerophilic, or CO₂-enriched) based on the suspected organism.
Laboratory Tips from the Bench
Always dry agar plates before dropping serial dilutions onto them — excess surface moisture causes drops to run together and ruins colony counts.
When a culture shows no growth, always double-check the incubation atmosphere before reporting a negative result — many false negatives are simply the wrong gas environment.
Remember the growth curve order with "Lazy Lions Sleep Deeply": Lag → Log → Stationary → Decline.
Important Notes
Spore-forming organisms typically sporulate during the stationary phase as nutrients deplete — a key reason spore-related tests (e.g. endospore staining) are best performed on older cultures, unlike Gram staining which is best on young (18–24 hour) cultures.
Ordinary laboratory cultures are "batch cultures" where growth stops once nutrients are depleted. Chemostats and turbidostats maintain continuous culture by constantly replenishing nutrients — used mainly in industrial and research settings.
Interactive Quiz
Test Your KnowledgeFlashcards
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Clinical Case Study
Apply Your KnowledgePresents with dysuria, urinary frequency and mild suprapubic discomfort for two days. Midstream urine sample sent for culture and sensitivity. Sample was collected at 9 AM but not received in the lab until 3 PM, having been left at room temperature.
Given E. coli's ~20-minute generation time, six hours of unrefrigerated transport allows for many rounds of exponential multiplication in the log phase, potentially inflating the reported colony count. The presence of pus cells supports a true infection, but the delayed transport is a pre-analytical error that should be flagged when interpreting the exact colony count.
- →Rapid bacterial generation time means even short transport delays can significantly alter quantitative culture results.
- →Urine specimens should be refrigerated (2–8°C) or processed within 1–2 hours of collection.
- →Pre-analytical errors must be documented and considered alongside quantitative culture results.
Frequently Asked Questions
The lag phase represents the time bacteria need to adapt to a new environment — synthesising the enzymes and metabolic intermediates required for cell division. Its duration depends on the species, inoculum size, medium composition, and environmental conditions such as temperature.
Total count measures all cells present (living and dead), typically by direct microscopic counting using counting chambers. Viable count measures only living cells capable of multiplication, obtained through dilution or plating methods that rely on colony formation.
Autotrophic bacteria can synthesise all essential organic compounds themselves and are of no medical importance. Many heterotrophic, medically important bacteria have lost or lack certain biosynthetic pathways and must obtain preformed compounds such as vitamins (growth factors) from their environment or host.
Quick Revision
10-Minute ReviewKey Takeaways
- Bacteria are unicellular prokaryotes that lack a nuclear membrane and organelles like mitochondria.
- Bacteria are classified nutritionally as phototrophs/chemotrophs and autotrophs/heterotrophs.
- Gaseous requirements divide bacteria into obligate aerobes, facultative anaerobes, obligate anaerobes and microaerophiles.
- Bacteria are classified by temperature preference as mesophilic, psychrophilic or thermophilic.
- Bacteria multiply by binary fission, with generation times ranging from 20 minutes to 20 days.
- The bacterial growth curve consists of lag, log, stationary and decline phases, each with distinct characteristics.
Competency Checklist
Track Your MasteryReferences
- NIOS Microbiology Module — Lesson 3: Nutrition and Growth of Bacteria.
- Ananthanarayan R, Paniker CKJ. Textbook of Microbiology.
- Miles AA, Misra SS. The estimation of the bactericidal power of the blood. J Hyg (1938).