Alkaline Peptone Water (APW): Composition, Principle, Preparation, and Uses in Vibrio Enrichment

During a cholera outbreak in a flood-affected district, a community health worker collects rectal swabs from five household contacts of a confirmed cholera case. None of them have symptoms — these are potential asymptomatic carriers. Their stool burden of Vibrio cholerae, if present, will be far lower than in an acute case shedding billions of organisms per milliliter of rice-water stool.

Direct plating onto TCBS agar from a low-count specimen risks a false-negative result — the few vibrio colonies present may be completely masked by residual coliform growth on the plate. The solution is a pre-enrichment step: inoculating the swab into Alkaline Peptone Water and incubating for 6–8 hours before TCBS plating. In that window, V. cholerae — adapted to alkaline environments — multiplies rapidly at the surface of the broth while competing intestinal flora are suppressed by the pH, giving the organism a decisive numerical advantage before any solid medium is used.

Overview

Alkaline peptone water (APW) is recommended as an enrichment broth for isolating Vibrio cholerae from clinical and non-clinical samples (suspected food & water samples). Many broth media have been described for the enrichment of V. cholerae, including Monsur’s enrichment medium and modification of APW with added potassium tellurite. Still, they may not offer a selective advantage over APW if used with a short incubation time (6 to 8 hours).

Enrichment media are liquid culture media which contain substances that inhibit the growth of unwanted organisms.

Enrichment broth should always be used with

Figure: Procedure for recovery ofVibrio choleraeO1 from a fecal sample

1. convalescent patients,
2. suspected asymptomatic infections,
3. environmental specimens, and
4. whenever high numbers of competing organisms are likely to present in the specimen.

but when the patient is in the very early stages of illness and is passing liquid stool, it may not be necessary to enrich stool specimens.

Principle

APW achieves its selective enrichment through two mechanisms acting simultaneously:

1. Alkaline pH (8.6–9.0) — selective suppression of competing flora: Most intestinal commensal bacteria — Escherichia coli, enterococci, Bacteroides, and other normal colonic flora — grow optimally at pH 7.0–7.4 and are significantly inhibited at pH 8.6–9.0. Vibrio cholerae is physiologically adapted to alkaline environments, reflecting its natural habitat in coastal waters, estuaries, and river systems at pH 8.0–9.6. This pH difference is the primary selective mechanism: APW creates an environment that is hostile to most commensals while being optimal for Vibrio growth.

2. Rapid Vibrio growth kinetics — enrichment by outcompeting: V. cholerae has an exceptionally short generation time of approximately 18–20 minutes under optimal conditions. In APW at 37°C, it multiplies so rapidly that it reaches detectable concentrations (>10⁵ CFU/mL) within 4–6 hours even from a very small initial inoculum. By the 6–8 hour subculture timepoint, V. cholerae will outnumber most surviving commensals by orders of magnitude.

The pellicle — biology of the surface growth pattern: After 6–8 hours of incubation, V. cholerae forms a thin film called a pellicle at the surface of the APW broth. This occurs because V. cholerae is both motile (single polar flagellum) and aerophilic (preferring oxygenated environments) — it actively swims toward the air-liquid interface where oxygen concentration is highest. Competing organisms that survive the alkaline pH but lack this directed motility settle toward the bottom of the tube.

This biology directly determines the subculture technique: always inoculate TCBS from the surface pellicle only, without shaking or mixing the broth. Mixing disrupts the spatial separation between the surface-enriched Vibrio and the sediment of competing organisms, reducing the sensitivity of the enrichment step.

When enrichment is and is not needed:

Composition of Alkaline Peptone Water

Final pH: 8.6 ± 0.2 at 25°C (adjusted with 1 mol/L sodium hydroxide)

Why only two ingredients? APW's simplicity is deliberate. It contains exactly enough nutrition to support rapid Vibrio growth but not enough to sustain the complex nutritional requirements of slower-growing commensals at alkaline pH. The alkaline pH does the selecting; the minimal peptone does the supporting. Adding enrichments (blood, serum, growth factors) would defeat the selective purpose.

Note on halophily: V. cholerae O1 and O139 are moderately halophilic — they require sodium chloride for optimal membrane function and growth. The 1% NaCl in APW is not just osmotic balance; it is a nutritional requirement that further selects for Vibrio over many non-halophilic intestinal flora.

Preparation of Alkaline Peptone Water

1. To make about 50 bottles (i.e. 500 ml), you will need 5 g of peptone and 5 g of Sodium chloride.
2. Dissolve the peptone and sodium chloride in the water.
3. Adjust the reaction of the medium to pH 8.6 to 9.0 using 1 mol/L sodium hydroxide.
4. Dispense the medium in 10 ml amounts in screw-cap bottles.
5. Sterilize by autoclaving (with caps loosened) at 121°C for 15 minutes. Tighten the bottle caps after the medium has cooled.
6. Date the medium and give it a batch number. Label the bottles and record on each the expiry date of the medium (2 years from preparation).
7. Store in a cool dark place with the bottle caps screwed tightly to prevent a change in pH.

Figure: Alkaline peptone water (apw) bottle

pH of the medium (pH 8.6 ± 0.2 @ 25°C)

Shelf-life: Up to 2 years providing there is no change in the volume or appearance of the medium suggesting contamination.

Uses

• Alkaline peptone water (APW) is an enrichment medium for Vibrio cholerae. Its alkalinity suppresses the growth of intestinal commensals, and V.cholerae grows rapidly. Prior enrichment in APW is unnecessary if the sample contains many vibrios (e.g., acute cholera).
• APW is a useful transport medium for Vibrio cholerae. APW is an excellent transport and enrichment medium for V. cholerae but unsuitable for other enteric pathogens.

APW vs Cary-Blair: Two Different Tools for Different Problems

APW and Cary-Blair are both used in cholera specimen management, but they serve entirely different purposes and must not be confused or used interchangeably.

The correct workflow combining both:

Field collection → Cary-Blair (transport to lab) → Lab receipt → Inoculate APW → 6–8 hrs incubation → Surface subculture to TCBS → 18–24 hrs incubation → Read colonies

When Cary-Blair is unavailable: APW may be used for transport, but only if the specimen will reach the laboratory and be subcultured within 6–8 hours of collection. Beyond this window, Proteus species (which are also alkaline-tolerant) begin to overgrow, and the enrichment advantage for Vibrio is lost. If >8-hour transport is anticipated and Cary-Blair is unavailable, the specimen should be refrigerated to slow all bacterial growth rather than being placed in APW.

Inoculation

APW can be inoculated with liquid stool, fecal suspension, or a rectal swab.

A. For use as an enrichment medium:

1. Inoculate the APW bottle with a specimen (The stool inoculums should not exceed 10% of the volume of the broth)
2. Incubate the tube with the cap loosened at 35°C to 37°C for 6 to 8 hours.
3. After 6 to 8 hours of incubation, subcultures to TCBS should be made with one to two loopful of APW from the surface and topmost portion of the broth, since vibrios preferentially grow in this area.
4. Do not shake or mix the tube before subculturing.
5. If the broth cannot be subcultured after 6 to 8 hours of incubation, subculture at 18 hours to a fresh tube of APW.
6. This second tube should be subcultured to a solid medium after 6 to 8 hours of incubation.

B. For use as a transport medium

1. When cholera is suspected, transfer about 1 ml of the specimen into 10 ml of sterile alkaline peptone water.
2. The inoculated medium should reach the laboratory within 8 hours of collection (Proteus species will grow eventually in alkaline peptone water.)

Important limitation — APW is Vibrio-specific: APW is optimised exclusively for Vibrio cholerae. Its highly alkaline pH (8.6–9.0) inhibits Salmonella, Shigella, Campylobacter, and virtually all other enteric pathogens. A specimen inoculated into APW for transport will lose all non-Vibrio enteric pathogens within hours. If a specimen must be transported for a broad enteric workup — not just cholera — use Cary-Blair transport medium, not APW.

Growth and further processing

Vibrio cholerae grows rapidly on APW, producing turbidity on and just below the surface of the medium within 4-6 hours. To confirm that the organisms are vibrios, examine a wet preparation/ perform hanging drop motility test, prepare gram stain of the smear (which may show gram-negative curved bacilli) and/or culture in the TCBS medium.

If the test for detecting Vibrio cholerae lipopolysaccharide antigen in the stool sample is done after enrichment in APW, its sensitivity and specificity increase.

How to Remember

APW = alkaline + peptone + water — three words, three functions:

• Alkaline (pH 8.6–9.0) → suppresses intestinal commensals; optimal for Vibrio
• Peptone → minimal nitrogen for Vibrio growth without enriching competing organisms
• Water → liquid medium enabling rapid Vibrio multiplication and pellicle formation

The pellicle as a visible enrichment endpoint: The appearance of a surface film (pellicle) in APW after 6–8 hours incubation is visual confirmation that Vibrio has grown. No pellicle does not necessarily mean no Vibrio — pellicle formation depends on inoculum size — but a visible pellicle is a positive sign. Always subculture from the surface regardless of whether a pellicle is visible, taking care not to disturb the bottom sediment.

The 6–8 hour window — why it closes:

• At 6–8 hours: Vibrio has maximally enriched relative to suppressed commensals → optimal subculture time
• At 18 hours: Proteus species, which tolerate alkaline conditions, recover and begin multiplying → competitive advantage for Vibrio is lost
• At 24+ hours: Multiple organisms competing; APW has lost selectivity

If the 6–8 hour window cannot be met, passage to a fresh APW tube (as described in the Inoculation section) resets the clock.

The cholera diagnostic chain — APW's position:

APW is the bridge between transport and selective plating — the step that makes the difference between detecting Vibrio at low counts and missing it entirely.

References and further readings:

1. Cheesbrough, M. (2006). District Laboratory Practice in Tropical Countries, Part 2 (2nd ed.). Cambridge University Press.
2. Centers for Disease Control and Prevention (CDC). Laboratory Methods for the Diagnosis of Vibrio cholerae. Atlanta: CDC. Available at: https://www.cdc.gov/cholera/laboratories.html
3. World Health Organization. (2004). Manual for the Laboratory Identification and Antimicrobial Susceptibility Testing of Bacterial Pathogens of Public Health Importance in the Developing World. Geneva: WHO.
4. Kaper, J. B., Morris, J. G., & Levine, M. M. (1995). Cholera. Clinical Microbiology Reviews, 8(1), 48–86.