Bacteria require specific nutrients for growth, and culture media are formulated to meet these nutritional needs. Understanding nutritional requirements and media types is essential for cultivating, isolating, and identifying microorganisms in the laboratory.
Nutritional Requirements
Macronutrients include carbon (the most abundant element, 50% of dry weight), nitrogen (for proteins and nucleic acids, 12–15% of dry weight), phosphorus (for nucleic acids and ATP), sulfur (for amino acids and cofactors), and minerals such as Mg²⁺, K⁺, Ca²⁺, and Fe²⁺. Micronutrients (trace elements) include iron, zinc, manganese, copper, cobalt, molybdenum, and nickel, required in micromolar or nanomolar concentrations for enzyme function. Growth factors are organic compounds that bacteria cannot synthesize, including vitamins (biotin, thiamine, B₁₂), amino acids, purines, and pyrimidines.
Nutritional Classification
Autotrophs use CO₂ as their sole carbon source; photoautotrophs obtain energy from light (cyanobacteria), while chemoautotrophs obtain energy from inorganic compounds (Nitrosomonas, Nitrobacter). Heterotrophs require organic carbon compounds — photoheterotrophs use light but require organic carbon (some purple bacteria), and chemoheterotrophs obtain both energy and carbon from organic compounds (most pathogenic bacteria). Prototrophs can synthesize all necessary organic compounds from simple carbon sources, whereas auxotrophs have lost the ability to synthesize one or more essential growth factors due to mutations.
Types of Culture Media
Complex media contain undefined ingredients such as peptones (enzymatic digests of protein), yeast extract, beef extract, and casein hydrolysate; nutrient agar, tryptic soy agar (TSA), and Luria-Bertani (LB) broth are common examples. Defined (synthetic) media have all components chemically defined and are used for nutritional studies and metabolic research — M9 minimal medium contains salts, glucose, and NH₄Cl. Enriched media are nutrient-rich media that support fastidious organisms; blood agar (TSA with 5% sheep blood) and chocolate agar (heated blood agar for Neisseria and Haemophilus) are examples.
Selective and Differential Media
Selective media contain inhibitors that suppress unwanted microorganisms while allowing target organisms to grow. MacConkey agar uses bile salts and crystal violet to inhibit Gram-positive bacteria, selecting for Gram-negative; mannitol salt agar uses 7.5% NaCl to select for staphylococci. Differential media distinguish between different types of microorganisms based on metabolic activity — MacConkey agar shows lactose fermenters as pink or red and non-fermenters as colorless, while EMB agar gives E. coli a green metallic sheen. Selective and differential properties are often combined in a single medium, as with MacConkey agar.
Specialized Media
Anaerobic media provide a reduced oxygen environment with reducing agents (cysteine, sodium thioglycolate) and resazurin as a redox indicator; cooked meat medium and thioglycolate broth support obligate anaerobes. Transport media maintain viability of delicate organisms during transport without promoting growth — Stuart’s medium and Cary-Blair medium preserve pathogens for clinical laboratory analysis. Indicator media contain substrates and indicators that produce a visible change in response to specific enzymatic activities; triple sugar iron (TSI) agar tests carbohydrate fermentation and H₂S production.
Sterilization of Media
Autoclaving at 121°C for 15 minutes is the standard method for sterilizing heat-stable media, while glucose-containing media are autoclaved at a reduced temperature (115°C) to prevent caramelization. Heat-sensitive media (e.g., those containing serum, antibiotics, or vitamins) are sterilized by membrane filtration with a 0.22 µm pore size. For quality control, each batch of medium is tested for sterility by incubation at 35°C for 48 hours and for growth support using reference strains (ATCC cultures).
Factors Affecting Growth in Culture
Temperature classification includes psychrophiles (−5 to 20°C), mesophiles (20–45°C, including most pathogens at 35–37°C), thermophiles (45–80°C), and hyperthermophiles (>80°C). Most bacteria grow optimally at pH 6.5–7.5, though acidophiles (e.g., Lactobacillus, Helicobacter) prefer pH 2–5 and alkaliphiles (e.g., Vibrio cholerae) prefer pH 8–10. Regarding oxygen requirements, obligate aerobes require O₂, obligate anaerobes are killed by O₂, facultative anaerobes grow with or without O₂, and microaerophiles require reduced O₂ (5–10%).
