Culture media and staining

In short

Culture media are nutrient preparations in which micro-organisms are grown, isolated, counted and identified in the laboratory, while staining techniques make the near-transparent, colourless bacterial cell visible under the light microscope and sort it into broad groups. The two go together: a colony on a plate tells you about growth requirements and biochemical behaviour, and a stained smear from that colony tells you shape, arrangement, cell-wall type and special structures. Every diagnostic microbiology lab in India — hospital labs under NABL (ISO 15189) accreditation, the RNTCP/NTEP TB laboratory network, FSSAI-notified food-testing labs, pharma QC under Schedule M and IP 2026 microbial-limit testing — runs on these two skills. University practical examinations, CSIR-UGC NET, GATE Biotechnology/Life Sciences (XL), ICMR-JRF and DBT-BET examine media classification and staining principles heavily, and campus interviews for QC microbiologist, medical lab technologist and research-associate roles almost always open with "tell me the Gram stain steps and why it works".

Explained

Culture media: composition and physical forms

• Definition: a medium is a solid, semi-solid or liquid preparation supplying water, a carbon and energy source, nitrogen, minerals, growth factors and a suitable pH (usually 7.2–7.4) for in-vitro multiplication.
• Agar: a galactose polymer (agarose + agaropectin) from red algae (*Gelidium*, *Gracilaria*). Used at 1.5–2% w/v; melts near 95–100 °C, sets near 42–45 °C, is not attacked by most bacteria and adds no nutrients. This wide melt–set gap is why it, and not gelatin, is the standard gelling agent.
• Peptone: partially digested protein (acid or enzymic hydrolysate of meat, casein or soya) supplying amino acids, peptides and some vitamins. Meat extract/beef extract supplies water-soluble carbohydrates, minerals and nucleotides. Yeast extract is rich in B-group vitamins.
• Physical forms: liquid (broth — no agar), semi-solid (0.2–0.5% agar, for motility), solid (1.5–2%), and biphasic (Castaneda medium for blood culture of *Brucella* and *Salmonella* Typhi).
• Preparation forms: plates (Petri dish, 20–25 mL per 90 mm dish), slants/slopes (for stock cultures), deeps/stabs (for anaerobes and motility), tubes of broth.
• Nature of ingredients: synthetic/defined media (every component and its quantity known — e.g. Davis minimal, Czapek-Dox; used for mutant and auxotroph work), semi-synthetic, and complex/undefined media (peptone, extracts — exact composition varies batch to batch).

Classification by purpose

1. Basal/simple media: nutrient broth and nutrient agar (peptone, beef extract, NaCl, water; agar for the solid form); peptone water; the base for most other media.
2. Enriched media: a basal medium plus added blood, serum, egg or vitamins for fastidious organisms. Blood agar = nutrient/soyabean casein digest agar + 5–10% sterile defibrinated sheep blood, added at 45–50 °C. Chocolate agar (heated blood agar) releases intracellular haemin (X factor) and NAD (V factor) needed by *Haemophilus influenzae*; *Neisseria gonorrhoeae* also needs it. Loeffler's serum slope for *Corynebacterium diphtheriae*.
3. Enrichment media (liquid): selectively favour one organism and suppress others so the wanted species outgrows the flora before plating — selenite F broth and tetrathionate broth for *Salmonella*/*Shigella* from faeces, alkaline peptone water (pH 8.6) for *Vibrio cholerae*. The distinction from "enriched" is a stock exam question: enriched = added nutrients, solid; enrichment = selective inhibition, liquid.
4. Selective media: contain inhibitors so only the target grows. Examples: MacConkey agar (bile salts + crystal violet inhibit Gram-positives), Deoxycholate citrate agar (DCA) for *Salmonella*/*Shigella*, TCBS for *V. cholerae*, Lowenstein–Jensen (LJ) medium with malachite green for *Mycobacterium tuberculosis*, Mannitol salt agar (7.5% NaCl) for staphylococci, Potato dextrose agar with antibiotics or Sabouraud dextrose agar (pH 5.6) for fungi, Thayer–Martin (VCNT antibiotics) for *Neisseria*, Wilson–Blair bismuth sulphite agar for *S.* Typhi.
5. Differential/indicator media: all organisms grow, but colonies look different because of a fermentable sugar plus a pH indicator or another indicator system. MacConkey agar (lactose + neutral red: lactose fermenters pink, non-fermenters pale) — both selective and differential. CLED agar (cystine-lactose-electrolyte-deficient, bromothymol blue) for urine, prevents *Proteus* swarming. XLD, Hektoen, SS agar, EMB agar (*E. coli* gives green metallic sheen), blood agar differentiates by haemolysis pattern.
6. Transport media: keep organisms viable without multiplication during transit — Cary–Blair (faeces, vibrios, campylobacter), Stuart's medium (gonococci), Venkatraman–Ramakrishnan (VR) medium for cholera stool, Amies with charcoal, buffered glycerol saline for enteric bacilli.
7. Sugar/biochemical media: peptone water sugars with Andrade's or bromothymol blue indicator and a Durham tube for gas; TSI agar, Hugh–Leifson O-F medium, Christensen's urease, SIM, MR-VP broth, Simmons citrate.
8. Anaerobic media: Robertson's cooked meat (RCM) broth (unsaturated fatty acids and glutathione lower Eh; meat particles also show saccharolytic/proteolytic action), thioglycollate broth, Wilkins–Chalgren and neomycin blood agar; anaerobic jar with GasPak, or anaerobic chamber. Methylene blue or resazurin indicator strips confirm anaerobiosis.
9. Assay and sensitivity media: Mueller–Hinton agar (MHA, pH 7.2–7.4, 4 mm depth) is the CLSI/ICMR AMR-surveillance standard for disc diffusion; MHA + 5% sheep blood for streptococci; antibiotic assay media for potency testing per IP.
10. Media for food, water and pharma: Soyabean casein digest medium (SCDM/TSB) and Fluid thioglycollate medium (FTM) for sterility testing per IP 2026/Ph. Eur. harmonised chapters; Baird–Parker (coagulase-positive staphylococci), Violet red bile glucose agar (Enterobacteriaceae), Plate count agar; m-Endo/ MI agar and MPN lactose broth for coliforms in water testing to IS 1622 methods, with drinking-water quality judged against IS 10500: 2012 (coliforms must be absent in 100 mL).

Culture methods, sterilisation of media and quality control

• Media preparation steps: weigh dehydrated powder → dissolve in distilled water with heat → adjust pH → dispense → autoclave at 121 °C, 15 psi (103 kPa), 15 min → cool to 45–50 °C → add heat-labile supplements (blood, serum, antibiotics, egg yolk) → pour/slant → check sterility by incubating a few plates at 35–37 °C for 24–48 h.
• Heat-sensitive media: LJ medium and Loeffler's slope are inspissated at 80–85 °C for 2 h (coagulates egg protein); sugars and urea are filter-sterilised through a 0.22 µm membrane; selenite F is boiled, not autoclaved.
• Inoculation techniques: streak plate (four-quadrant or primary-secondary streaking with flaming between sets) for pure isolation; pour plate and spread plate for counting; stab, lawn/carpet culture (for sensitivity and phage typing), stroke culture on slants.
• Incubation: 35–37 °C aerobically for most pathogens; 5–10% CO₂ (candle jar) for pneumococci and *Brucella*; 42 °C for *Campylobacter*; 25 °C for fungi; LJ slopes for 6–8 weeks.
• Colony morphology recorded as: size, shape, elevation, margin, surface, opacity, colour/pigment, consistency, odour, haemolysis, and emulsifiability.
• QC of each batch: sterility check, growth-promotion and inhibitory tests with reference strains (ATCC/MTCC — the MTCC national culture collection is at CSIR-IMTECH Chandigarh), pH check after setting, gel strength, and depth. NABL-accredited labs must document every batch; Schedule M (revised, with staged compliance for smaller units) and WHO-GMP require media growth-promotion testing and media-fill validation.

Staining: principles and the simple/negative stains

• Why stain: bacterial cytoplasm has almost the same refractive index as water, so an unstained cell has very low contrast under bright-field optics.
• Dye chemistry: dyes are salts with a chromophore (colour-bearing group) and an auxochrome (salt-forming group). Basic dyes (methylene blue, crystal violet, basic fuchsin, safranin, malachite green) are cationic and bind the negatively charged bacterial surface (teichoic acid, phospholipid, nucleic acid) — hence most bacterial stains are basic. Acidic dyes (eosin, acid fuchsin, nigrosin, India ink, Congo red) are anionic and stain the background or cytoplasmic proteins at low pH.
• Mordant: a substance that fixes or intensifies the dye by forming a larger insoluble complex — iodine in the Gram stain, tannic acid in flagellar staining.
• Fixation: heat fixation (passing the air-dried smear through a flame 2–3 times) kills cells, coagulates protein and makes the smear stick; methanol fixation (1 min) is used for blood films and preserves morphology better.
• Smear making: a thin, even film; too thick a smear is the commonest cause of a false Gram-positive result.
• Simple stain: one dye (Loeffler's methylene blue, dilute carbol fuchsin, safranin) — shows shape, size and arrangement only.
• Negative stain: India ink or nigrosin stains the background; the cell and its capsule stand out unstained — classically used for *Cryptococcus neoformans* capsule in CSF.
• Impression/relief staining: nigrosin or India ink wet mount for spirochaetes and capsules; dark-ground microscopy is preferred for *Treponema pallidum*.

Differential stains in detail

Gram stain (Christian Gram, 1884)

1. Primary stain: crystal violet (or methyl violet/gentian violet) 30–60 s.
2. Mordant: Gram's iodine 30–60 s → forms a large crystal violet-iodine (CV-I) complex inside the cell.
3. Decolouriser: acetone (seconds), ethanol 95% (10–30 s) or acetone-alcohol.
4. Counterstain: safranin, dilute carbol fuchsin or neutral red 30–60 s.
5. Result: Gram-positive = purple/violet; Gram-negative = pink/red.
6. Mechanism: the Gram-positive wall has a thick (20–80 nm), highly cross-linked peptidoglycan with teichoic acids and very little lipid; alcohol dehydrates and shrinks the pores so the CV-I complex is retained. The Gram-negative wall has thin peptidoglycan (2–7 nm) and a lipid-rich outer membrane that alcohol dissolves, opening the wall and washing the complex out.
7. Critical step: decolourisation — over-decolourisation makes Gram-positives look negative; under-decolourisation does the reverse.
8. Use a 18–24 h culture: old Gram-positive cultures lose peptidoglycan integrity and appear Gram-variable.
9. Gram-positive: *Staphylococcus*, *Streptococcus*, *Bacillus*, *Clostridium*, *Corynebacterium*, *Listeria*, *Mycobacterium* (weakly, acid-fast). Gram-negative: *Escherichia*, *Salmonella*, *Shigella*, *Klebsiella*, *Pseudomonas*, *Vibrio*, *Neisseria*, *Haemophilus*. Gram-non-stainable: mycoplasma (no wall), chlamydia, rickettsia, spirochaetes, mycobacteria (poorly).

Acid-fast / Ziehl–Neelsen stain (ZN)

• Reagents: carbol fuchsin (basic fuchsin in 5% phenol) with heating until steam rises for 5 min (phenol and heat drive the dye through the waxy wall), decolourise with 20–25% sulphuric acid (1% sulphuric acid for *Nocardia*; 5% for *M. leprae*), counterstain methylene blue (or malachite green).
• Result: acid-fast bacilli = bright red rods on a blue background; everything else blue.
• Basis: mycolic acid (long-chain β-hydroxy fatty acids) in the cell wall plus the integrity of the wall; the dye-mycolate complex resists acid.
• Cold method — Kinyoun's uses more phenol and no heat.
• NTEP practice: ZN sputum smear microscopy is graded (scanty, 1+, 2+, 3+) per the national TB programme manual; fluorescent (auramine-O/rhodamine) LED microscopy is now the recommended first-line smear method in designated microscopy centres, with NAAT (Truenat, CBNAAT) for rapid diagnosis and DST.
• Other acid-fast objects: *Nocardia* (partially), *Mycobacterium leprae*, oocysts of *Cryptosporidium* and *Cyclospora* (modified ZN), bacterial spores (weakly).

Special stains

• Capsule: negative staining (India ink), Maneval's, Anthony's method (crystal violet + 20% copper sulphate), M'Fadyean's methylene blue for *Bacillus anthracis* in blood films.
• Spore: Schaeffer–Fulton — malachite green with heat, water wash, safranin counterstain → green spore in a red/pink vegetative cell. Dorner's method uses carbol fuchsin and nigrosin.
• Flagella: Leifson's method, or silver impregnation (Ryu/Fontana) using tannic acid as mordant to thicken the flagellum beyond the resolution limit.
• Metachromatic granules (volutin): Albert's, Neisser's or Ljubinsky's stain for *C. diphtheriae* — green bacilli with bluish-black granules, Chinese-letter/cuneiform arrangement.
• Cell wall and intracellular structures: Dyar's method; Sudan black for lipid (PHB) inclusions; Lugol's iodine for glycogen; Feulgen for DNA; Loeffler's methylene blue for granules.
• Spirochaetes: Fontana's silver impregnation; Giemsa/Wright for borreliae and blood parasites.
• Fungi: lactophenol cotton blue (LPCB) mount, KOH 10–20% mount for skin scrapings, PAS and GMS in histology, calcofluor white fluorescence.
• Fluorescent staining: auramine-rhodamine (mycobacteria), acridine orange (nucleic acid, viable cells in low-count specimens), FITC-conjugated antibody for direct immunofluorescence (rabies, *Pneumocystis*), DAPI for DNA in research labs.

Viability, counting and the limits of culture

• Microscopy counts (Breed's, haemocytometer, DEFT) count total cells including dead ones; culture counts (CFU) count only cells able to form a colony on that medium under those conditions.
• Viable but non-culturable (VBNC) cells and fastidious/unculturable organisms mean culture underestimates true diversity — the reason molecular methods (16S rRNA sequencing, qPCR, MALDI-TOF for identification) now supplement culture in Indian tertiary labs.
• Great plate count anomaly: plate counts of an environmental sample typically recover only about 0.1–1% of microscopically visible cells.
• Biosafety: media and stained-smear work on pathogens follows BSL-2 practice (BSL-3 for *M. tuberculosis* culture); discard goes into autoclavable bags and is handled under the Bio-Medical Waste Management Rules, 2016 (as amended) — laboratory cultures and stocks must be pre-treated/disinfected on site before disposal (yellow category).

Key points

• Agar — 1.5–2% algal polysaccharide gelling agent; melts ~95 °C, sets ~42–45 °C, not digested by most bacteria, nutritionally inert.
• Basal medium — simple nutrient broth/agar (peptone + beef extract + NaCl ± agar) used as the base of other media.
• Enriched vs enrichment — enriched = added nutrients (blood agar); enrichment = liquid, selectively inhibits competitors (selenite F).
• Selective medium — inhibitor stops unwanted flora (MacConkey's bile salts, LJ's malachite green, MSA's 7.5% NaCl).
• Differential medium — sugar + indicator makes colonies distinguishable (MacConkey: pink lactose fermenters vs pale non-fermenters).
• Chocolate agar — blood agar heated to 70–80 °C; releases X (haemin) and V (NAD) factors for *Haemophilus*.
• Transport media — maintain viability without growth: Cary–Blair, Stuart's, VR medium, Amies.
• Robertson's cooked meat broth — all-purpose anaerobic medium; meat particles lower the redox potential.
• Mueller–Hinton agar — standard disc-diffusion medium, 4 mm depth, pH 7.2–7.4, as per CLSI used in ICMR AMR surveillance.
• Inspissation — 80–85 °C for 2 h, used for egg-based LJ medium and serum slopes, instead of autoclaving.
• Basic dyes are cationic and stain the negatively charged cell; acidic dyes stain the background or acidic cell contents.
• Mordant — fixes/intensifies the dye (iodine in Gram stain, tannic acid in flagellar stain).
• Gram stain order — crystal violet → iodine → acetone/alcohol → safranin; positive purple, negative pink.
• Gram mechanism — thick cross-linked peptidoglycan retains the CV-iodine complex; lipid-rich outer membrane of Gram-negatives is dissolved by alcohol.
• Acid-fastness — mycolic acid in the wall retains carbol fuchsin against 20–25% H₂SO₄; AFB appear red on blue.
• Schaeffer–Fulton — green spore inside a pink vegetative cell (malachite green + safranin).
• Albert's stain — metachromatic granules of *C. diphtheriae*; green bacilli with black granules in Chinese-letter arrangement.
• Negative staining — India ink/nigrosin outlines capsules, famously *Cryptococcus* in CSF.
• CFU vs total count — culture counts only culturable cells; plate counts recover roughly 0.1–1% of an environmental population.

Formulas and examples

Example 1 — Viable count by spread plate. A water sample is diluted 1 mL into 9 mL diluent three times (10⁻¹, 10⁻², 10⁻³), then 0.1 mL of the 10⁻³ tube is spread on plate count agar. After 48 h at 35 °C, 164 colonies are counted.

• Total dilution of the tube plated = 10⁻³ → dilution factor = 1,000.
• Volume plated = 0.1 mL.
• CFU/mL = 164 × 1,000 / 0.1 = 1.64 × 10⁶ CFU/mL.
• Check: countable range is 30–300 colonies, so 164 is a valid plate; the 10⁻² plate would have been >1,000 (uncountable) and the 10⁻⁴ plate about 16 (below range).

Example 2 — Pour-plate count of milk against a specification. 1 mL of a 10⁻⁴ dilution of pasteurised milk is pour-plated; 48 colonies grow.

• CFU/mL = 48 × 10,000 / 1 = 4.8 × 10⁵ CFU/mL.
• Standard-plate-count limits for pasteurised milk under the FSSAI food-product-standards regulations are of the order of 3 × 10⁴ CFU/mL at the point of manufacture (the exact limit depends on the product category and the current amendment, so the notified table must be checked), so this sample would fail by more than an order of magnitude.

Example 3 — Media preparation arithmetic. Prepare 1.5 L of nutrient agar (label dose 28 g/L) and 500 mL of blood agar with 7% sheep blood.

• Nutrient agar powder = 28 × 1.5 = 42 g in 1,500 mL distilled water.
• Plates obtainable at 22 mL/plate = 1,500 / 22 ≈ 68 plates.
• Blood needed = 500 × 7/100 = 35 mL sterile defibrinated sheep blood, added after cooling the molten base to 45–50 °C.
• If agar-agar had to be added separately to a 2% final concentration in that 500 mL: 500 × 2/100 = 10 g.

Example 4 — Gram-stain decolourisation timing. A technician reports *Staphylococcus aureus* from pus as Gram-negative cocci.

• Smear thickness was normal, counterstain was fresh, so suspect the decolouriser: acetone acts in 2–5 s whereas 95% ethanol needs 10–30 s. The technician used acetone for 20 s → over-decolourisation stripped the CV-iodine complex.
• Corrective action: repeat with a 24 h culture, decolourise acetone-alcohol for ≤5 s, and run a positive (*S. aureus*) and negative (*E. coli*) control smear on the same slide.

Comparison tables

Where it is used

• Hospital and diagnostic labs: blood, urine, sputum, pus and CSF culture on blood/MacConkey/chocolate/CLED agar with Gram and ZN smears — roles of medical microbiologist and DMLT/BMLT technologist in NABL ISO 15189 labs (Apollo, Fortis, AIIMS, PGIMER, SRL, Metropolis, Dr Lal PathLabs).
• National TB programme (NTEP): designated microscopy centres run ZN and LED-fluorescence sputum smears; culture and drug-susceptibility testing on LJ and liquid MGIT media in accredited C&DST labs, with Truenat/CBNAAT for rapid NAAT.
• Pharmaceutical QC: sterility testing (SCDM and FTM), microbial-limit tests, bioburden, environmental monitoring settle/contact plates and antibiotic potency assays per IP 2026 and revised Schedule M of the Drugs and Cosmetics Rules — QC microbiologist roles at Sun Pharma, Cipla, Dr Reddy's, Biocon, Serum Institute.
• Food and dairy industry: standard plate count, coliform, *Salmonella*, *Listeria* and yeast-mould counts in FSSAI-notified labs, Amul/Mother Dairy/Nestlé/ITC plants, following FSSAI manuals and IS/ISO methods; water testing to IS 10500: 2012 limits.
• Agricultural and industrial microbiology: rhizobial and biofertiliser inoculant counts on YEMA (quality standards under the Fertiliser Control Order), starter-culture maintenance in fermentation and brewing, MTCC/NCCS culture-collection work.
• Research and academia: defined minimal media for auxotroph selection and molecular cloning (LB, M9, antibiotic-selection plates), environmental and soil microbiology, CSIR lab and university technician/JRF positions.

Interview questions

• Q: Why is agar preferred over gelatin as a solidifying agent? A: It melts near 95 °C but solidifies only at about 42–45 °C, is not hydrolysed by most bacteria and is nutritionally inert, so plates stay solid at 37 °C and the medium's composition is unchanged.
• Q: What is the difference between an enriched and an enrichment medium? A: An enriched medium is usually solid and has extra nutrients added (blood agar) for fastidious organisms; an enrichment medium is liquid and contains inhibitors so the wanted organism outgrows the competing flora (selenite F broth).
• Q: MacConkey agar is called both selective and differential — why? A: Bile salts and crystal violet select against Gram-positives, while lactose with neutral red differentiates fermenters (pink colonies) from non-fermenters (pale colonies).
• Q: Explain the chemical basis of the Gram stain. A: Crystal violet and iodine form a large insoluble complex inside the cell; the thick, cross-linked peptidoglycan of Gram-positives shrinks on alcohol treatment and traps it, while the lipid-rich outer membrane of Gram-negatives is dissolved and the complex washes out.
• Q: Which is the most critical step in Gram staining and why? A: Decolourisation — too long and Gram-positives read as negative, too short and Gram-negatives read as positive; it is also why a fresh 18–24 h culture and a thin smear are essential.
• Q: Why are mycobacteria acid-fast? A: Their wall contains mycolic acids and waxes that bind the carbol fuchsin-phenol complex and resist removal by 20–25% sulphuric acid, so they stay red while other cells are decolourised.
• Q: Why is heat used in Ziehl–Neelsen but not in Kinyoun's method? A: Heat drives the dye through the waxy wall; Kinyoun's compensates with a higher phenol and fuchsin concentration, so it works cold.
• Q: Why can't *Mycoplasma* be Gram-stained usefully? A: It has no peptidoglycan cell wall, so there is nothing to retain or exclude the dye complex, and the cells are also too small to resolve reliably.
• Q: How is chocolate agar made and what does it supply? A: Blood agar is heated to about 70–80 °C, lysing the red cells to release haemin (X factor) and NAD (V factor) required by *Haemophilus influenzae*.
• Q: Why is blood added at 45–50 °C and not to boiling medium? A: Higher temperatures lyse the cells and denature proteins, converting it into chocolate agar and destroying the haemolysis reactions you need to read.
• Q: Which medium is sterilised by inspissation and why? A: Lowenstein–Jensen medium and serum slopes — autoclaving would denature the egg albumin and serum, so heating at 80–85 °C for 2 h coagulates and sterilises it instead.
• Q: A plate gives 400 colonies at 10⁻² and 3 at 10⁻⁴. Which do you report? A: Neither directly — both are outside the 30–300 countable range; repeat with a 10⁻³ dilution, and if forced to report, flag the result as estimated.
• Q: What does a negative stain show that a simple stain cannot? A: The capsule, because the acidic dye cannot penetrate it or the cell, so the capsule appears as a clear halo against a dark background.

Common mistakes

• Confusing "enriched" with "enrichment" media, or calling MacConkey agar purely selective when it is also differential.
• Over-decolourising the Gram stain, or using an old (>48 h) culture, then reporting a Gram-positive organism as Gram-negative.
• Making a thick, uneven smear, or heat-fixing before the smear is air-dried (distorts cells and bakes on debris).
• Adding blood, serum, antibiotics or urea to medium that has not cooled to 45–50 °C, or autoclaving egg-based and selenite media.
• Writing the Gram counterstain as crystal violet, or swapping the ZN counterstain (methylene blue) with safranin in the answer.
• Reporting plate counts from plates outside the 30–300 range, forgetting to multiply by the dilution factor, or dividing by the plated volume incorrectly (0.1 mL spread plates are a factor-of-10 trap).
• Omitting positive and negative control smears and uninoculated sterility-check plates — a documented requirement in NABL and GMP labs.

Related topics

Bacterial structure and classification · Sterilisation and disinfection · Bacterial growth and metabolism · Microbial genetics · Biochemical tests and bacterial identification · Antimicrobial susceptibility testing and AMR