Culturing Microorganisms in the Laboratory

A culture medium is an artificial nutrient environment, either liquid (broth) or solidified with a gelling agent (agar plate or slant), formulated to support microbial growth outside the organism's natural habitat. Media are described along three independent axes. By chemical composition, a medium is either defined (synthetic) — every ingredient and its exact concentration known, e.g., glucose, ammonium salts, phosphate buffer, trace metals — or complex (undefined) — built from digests of yeast, meat, or soybeans (peptone, tryptone, yeast extract) whose exact chemical makeup varies batch to batch. By selectivity, a medium can be general-purpose (supports many species), selective (contains an agent that inhibits unwanted organisms while permitting the target to grow, such as bile salts suppressing Gram-positive bacteria), or differential (does not inhibit growth but makes some colonies visually distinguishable from others, typically via a pH indicator or a fermentable sugar). By purpose, an enrichment culture uses a liquid medium and specific incubation conditions — a particular carbon source, temperature, pH, oxygen tension, light regime — to shift the competitive balance of a mixed natural sample toward a rare organism, increasing its relative abundance before it is ever plated. A pure culture (also called an axenic culture) contains a single microbial species, ideally all descended from one progenitor cell — a clone. This single-cell origin matters enormously: it means every cell in the population shares essentially the same genotype (barring new mutations), so physiological, biochemical, and genetic experiments performed on the culture describe one organism, not an ensemble average of several. The entire discipline of physiological microbiology — determining an organism's optimal temperature, its fermentation products, its antibiotic susceptibility — depends on this single-species starting condition being achieved and verified. The tension running through this whole subject is that "grows well in pure culture on standard media" is not a universal property of bacteria — it is a property of the small subset of bacteria whose natural nutritional and physical requirements happen to be easy to reproduce artificially. Fast-growing copiotrophs like Escherichia coli, adapted to nutrient-rich mammalian guts, thrive on rich broth. Slow-growing oligotrophs adapted to nutrient-poor lakes, deep subsurface rock, or symbiotic partnerships may starve, or be outcompeted by faster neighbors, the moment they are given rich medium — the opposite of what intuition suggests.

Building a medium. Every medium must supply a carbon and energy source, a nitrogen source, essential minerals (phosphorus, sulfur, potassium, magnesium, trace metals), and, for many organisms, specific vitamins or growth factors the cell cannot synthesize itself. A defined medium for E. coli might be as simple as glucose, ammonium chloride, dipotassium phosphate, magnesium sulfate, and water — five or six ingredients supporting full growth because E. coli can biosynthesize everything else it needs. A complex medium like nutrient broth (peptone + beef extract) or Luria-Bertani broth (tryptone + yeast extract + NaCl) supplies amino acids, vitamins, and nucleotide precursors already broken down, supporting the growth of fastidious organisms that cannot make these compounds themselves, at the cost of not knowing precisely what the cells are consuming.

Solidifying the medium. Robert Koch's laboratory, in the 1880s, needed a way to grow bacteria as physically isolated colonies rather than as a uniform turbid broth. Gelatin was tried first but had two fatal flaws: many bacteria secrete gelatinases that liquefy it, and it melts at approximately 25°C, well below the 37°C optimal for most pathogens. The solution came from Fannie Eilshemsz Hesse, wife of Koch's assistant Walther Hesse, who suggested agar-agar, a polysaccharide extracted from red algae that she used to set jams. Agar melts near 100°C but does not resolidify until it cools to about 40°C, and almost no bacteria can metabolize it. This wide hysteresis lets a microbiologist pour molten agar at ~45°C (cool enough not to kill cells mixed into it) and have it remain solid at incubation temperatures up to 100°C above its resolidification point.

Isolating single cells — the streak plate. The streak-plate method, developed by Koch, physically dilutes a mixed sample across the surface of an agar plate using a flame-sterilized inoculating loop. A small sample is streaked in one section, the loop is re-sterilized, and a new stroke picks up a smaller inoculum from the edge of the previous streak and drags it into a fresh, unstreaked area — typically across three or four sequential sectors. Each successive streak further dilutes the cell density, so that by the final sector, individual cells land far enough apart that each grows into its own visually distinct colony, a macroscopic clone founded by one cell. Picking a well-isolated colony and restreaking it confirms purity.

Isolating single cells — the spread plate and pour plate. A spread plate delivers a measured, serially diluted volume of liquid culture onto the surface of solid agar, spreading it with a sterile glass or plastic rod so surviving cells are statistically distributed far apart. A pour plate mixes a diluted sample directly into molten (cooled) agar before it solidifies, so colonies form both on the surface and embedded within the gel. Both methods pair naturally with serial dilution — sequential 1:10 transfers of sample into sterile diluent — which is necessary because environmental and clinical samples usually contain far too many cells per milliliter to separate into distinct colonies without first thinning them out mathematically.

Enrichment and selection working together. Enrichment culture exploits Aerobic vs Anaerobic Respiration and Fermentation and specific nutrient chemistry to bias a mixed community before isolation is even attempted. To recover a bacterium that degrades an obscure herbicide, a microbiologist inoculates soil into a minimal liquid medium where that herbicide is the only carbon source; only organisms with the right catabolic enzymes can grow, and repeated transfer into fresh identical medium progressively dilutes out everyone else. Winogradsky's and Beijerinck's late-19th-century enrichment cultures for nitrifying bacteria, sulfur bacteria, and nitrogen fixers established this logic and remain the template for isolating novel metabolic specialists today.

Enumeration once a culture exists. Different questions demand different counting methods, and they do not measure the same thing:

The viable plate count relies on the assumption that each colony arises from a single viable cell or a single cell cluster — hence the unit colony-forming unit (CFU) rather than "cell," acknowledging that clumps or chains can inflate or bias raw counts. A sample is serially diluted (say, ten tenfold dilutions), a fixed volume of several dilutions is spread-plated, and after incubation the dilution yielding between roughly 30 and 300 colonies per plate — the statistically reliable range, avoiding both undercounting from overcrowding and the noise of very low counts — is used to back-calculate the original concentration: CFU/mL = (colonies counted) ÷ (volume plated × dilution factor) For example, 214 colonies on a plate from the 10⁻⁶ dilution, with 0.1 mL plated, gives 214 ÷ (0.1 × 10⁻⁶) = 2.14 × 10⁹ CFU/mL in the original sample.

Koch's proof that pure culture on solid media reveals causation (1876–1884). Before Koch, bacteria were studied almost exclusively in liquid broth, where mixed populations could not be separated and where it was nearly impossible to prove that a specific microbe, rather than some contaminant, caused a specific disease. Working first with Bacillus anthracis, the anthrax bacillus, Koch grew the organism through multiple serial transfers in a defined liquid medium (aqueous humor from ox eyes), demonstrating that the bacillus alone, without any host tissue, could still form spores and retain virulence — evidence that the microbe itself, not some vague host-derived "miasma," was the causative agent. He then extended this logic using solid media (initially cut potato slices, then gelatin, then Fannie Hesse's agar) to physically separate single bacterial cells into isolated colonies, proving each colony was clonal by re-inoculating it and reproducing identical morphology, staining behavior, and pathogenicity every time. This directly enabled the Henle-Koch postulates: the organism must be found in all cases of the disease, must be isolated in pure culture, must reproduce the disease when introduced into a healthy host, and must be re-isolated from that host, unchanged. Applying this framework to Mycobacterium tuberculosis in 1882 and Vibrio cholerae in 1883–84, Koch conclusively identified the causative agents of tuberculosis and cholera — landmark results that could not have been obtained from turbid mixed broths, only from the isolated, verifiable clones that solid-medium plating made possible.

Winogradsky and Beijerinck: enrichment culture reveals metabolic diversity invisible to Koch's methods (1880s–1900s). Sergei Winogradsky, studying sulfur bacteria and later nitrifying bacteria, and Martinus Beijerinck, studying nitrogen-fixing and sulfate-reducing bacteria, showed that supplying a single specific substrate as the sole energy or nitrogen source in liquid enrichment culture could select rare specialists out of complex soil or water communities that standard rich media never revealed. Winogradsky's demonstration that Nitrosomonas and Nitrobacter could grow with no organic carbon at all, oxidizing ammonia or nitrite as their sole energy source in the dark, established chemolithotrophy as a real mode of metabolism and could only be shown by matching the medium precisely to the organism's actual physiology — a direct forerunner of the modern insight that "unculturable" often just means "medium doesn't match physiology."

Staley and Konopka (1985): quantifying the great plate count anomaly. James Staley and Allan Konopka directly compared, for freshwater lake samples, the total cell count obtained by epifluorescence direct microscopy against the viable count obtained by standard plating on nutrient agar. They found that plate counts recovered only about 0.001% to 1% of the cells visible microscopically — meaning that for every culturable cell counted on a plate, somewhere between roughly 100 and 100,000 morphologically similar cells in the same water sample never formed a colony at all. This was not a new observation in isolated form (microbiologists had long noted plate counts run low), but Staley and Konopka's systematic, quantitative comparison across environments gave the phenomenon its name — the great plate count anomaly — and made it a central, quantifiable research problem rather than an anecdotal caveat. Subsequent work using 16S rRNA gene sequencing directly from environmental DNA, without any culturing step, confirmed that the uncultured majority represents not just quiescent members of already-known species but entire phyla and candidate divisions with no cultured representative at all, some later cultivated only decades afterward using radically modified approaches (diffusion chambers, co-culture with helper strains, extremely dilute media, months-long incubations).

Example 1 — Choosing and reading a selective-differential medium in a clinical lab. A stool sample from a patient with suspected bacterial gastroenteritis is plated onto MacConkey agar, a medium that is both selective (bile salts and crystal violet suppress most Gram-positive organisms, favoring Gram-negative enteric bacteria) and differential (it contains lactose and a pH indicator, neutral red, that turns colonies pink-red if the organism ferments lactose and produces acid, while non-fermenters stay pale/colorless). After 18 hours at 37°C, the plate shows a mix of large pink mucoid colonies and smaller pale colorless colonies. The pink colonies are presumptively lactose-fermenting commensals such as E. coli, common gut flora and not automatically pathogenic. The pale, non-fermenting colonies are the ones flagged for further workup, because major enteric pathogens like Salmonella and Shigella characteristically do not ferment lactose. The microbiologist picks a pale colony, restreaks it to confirm purity, and proceeds to biochemical and serological identification — the differential color reaction did not identify the pathogen outright, but it triaged which of dozens of colonies deserved further, more expensive testing. This illustrates why selective/differential media are the backbone of diagnostic bacteriology: they convert an intractable mixed sample into a short, prioritized list of candidates in a single overnight step.

Example 2 — Calculating culturability from a lake water sample (the plate count anomaly in numbers). A limnologist collects 1 mL of lake water and performs two counts in parallel. Under an epifluorescence microscope using a nucleic acid stain and a calibrated counting grid, she counts an average of 42 fluorescent cells per grid square, where each grid square corresponds to a sample volume of 2 × 10⁻⁷ mL. Total direct count = 42 ÷ (2 × 10⁻⁷ mL) = 2.1 × 10⁸ cells/mL. She also serially dilutes the same water tenfold six times and spread-plates 0.1 mL from the 10⁻³ dilution onto nutrient agar; after 5 days of incubation at 20°C, 61 colonies appear. Viable count = 61 ÷ (0.1 mL × 10⁻³) = 6.1 × 10⁵ CFU/mL. The ratio of viable count to total direct count is 6.1 × 10⁵ ÷ 2.1 × 10⁸ ≈ 0.0029, or about 0.29% culturability — meaning fewer than 3 in every 1,000 cells visible under the microscope produced a colony on this particular medium under these particular conditions. This does not mean 99.7% of the cells were dead; independent viability stains (e.g., distinguishing intact from compromised membranes) on similar samples typically still show the majority of cells metabolically active. The gap instead reflects a mismatch between the artificial medium/incubation conditions offered and the real physiological requirements — oligotrophic growth rates, unusual nutrient needs, syntrophic dependence on neighboring species, or simple intolerance of the oxygen tension and nutrient concentration of standard nutrient agar.

Robert Koch trained as a physician in rural Prussia and did much of his foundational bacteriology work as a district medical officer in Wollstein, using a homemade laboratory and his own improvised microscopy and photography techniques before ever holding a formal research post. He is remembered chiefly for identifying the causative agents of anthrax, tuberculosis, and cholera, but his more durable legacy for the discipline as a whole was methodological: the introduction of solid culture media for isolating pure clones, the systematic use of aniline dyes and photomicrography to document bacterial morphology, and the postulates that gave "proving causation" a testable, repeatable structure. He shared the 1905 Nobel Prize in Physiology or Medicine for his tuberculosis work and trained or influenced a generation of microbiologists, including several who went on to identify the causative agents of diphtheria, tetanus, and typhoid using the exact isolation techniques he had pioneered.

Clinical microbiology laboratories still run selective and differential media as first-line diagnostic tools — blood agar to detect hemolysis patterns, MacConkey and eosin-methylene blue agar to sort Gram-negative enterics, mannitol salt agar to select for and flag Staphylococcus aureus — because a same-day colony morphology and color reaction remains cheaper and faster than sequencing for routine identification, even as Antibiotics and Their Mechanisms of Action and rapid molecular diagnostics increasingly supplement it. Food and water safety testing relies on standardized viable counts and selective media (e.g., media selective for coliforms and for E. coli specifically) to certify products against regulatory limits, with the countable-colony-window logic embedded directly into official testing protocols. Industrial biotechnology depends on pure, well-characterized cultures — a single verified strain of Saccharomyces cerevisiae, E. coli, or Corynebacterium glutamicum — to reliably manufacture insulin, biofuels, amino acids, and antibiotics at scale, since a contaminated or mixed culture in a bioreactor can derail an entire production batch. Conservation and environmental microbiology increasingly pursue the previously unculturable majority directly, using diffusion-chamber devices that let cells grow in situ surrounded by natural sediment or soil while separated from it by a membrane, or dilution-to-extinction culturing in oligotrophic media that mimic ocean or soil nutrient scarcity rather than force-feeding rich broth — techniques that have successfully cultivated organisms from phyla with no prior cultured representative, closing a small but growing fraction of the plate count anomaly and connecting genomic surveys back to living, testable organisms.

Ask the tutor: Why might an organism that grows perfectly well in its natural environment die or fail to grow at all when given a "rich," nutrient-dense laboratory medium?