Yes, Proteus vulgaris grows on MacConkey agar. It comes up as colorless to off-white colonies because it does not ferment lactose, which means there is no acid drop to trigger the neutral red indicator. The bile salts in the medium also tend to suppress the characteristic swarming you would normally see on plain nutrient agar or blood agar, so the colonies are more compact and easier to read than you might expect from a Proteus species. On mannitol salt agar, Proteus species would not be expected to grow like true salt-tolerant organisms, because the medium favors bacteria that can both tolerate high salt and ferment mannitol.
Does Proteus vulgaris grow on MacConkey agar?
Marcus Reeves
27 May 2026
What MacConkey agar is actually doing
MacConkey is a selective and differential medium. The selectivity comes from crystal violet (0.001 g/L) and bile salts (around 1.5 g/L), which together inhibit most Gram-positive bacteria so only Gram-negatives establish colonies. MacConkey agar suppresses most Gram-positive bacteria because of crystal violet and bile salts, so Gram-positive growth is generally not expected on this medium can gram-positive bacteria grow on MacConkey agar. The differentiation comes from lactose (10 g/L) and neutral red (0.03 g/L). When a bacterium ferments lactose, it produces organic acids that lower the local pH, and neutral red turns pink to red at that lower pH. Bacteria that ignore lactose produce no acids, so the neutral red stays its pale straw color and colonies appear colorless or translucent.
This setup makes MacConkey useful for sorting Gram-negative isolates quickly: you get a rough first split between lactose fermenters and non-fermenters on the same plate, which narrows down the identification considerably before you even run confirmatory tests.
Proteus vulgaris on MacConkey: what to expect
Proteus vulgaris is Gram-negative and grows well on MacConkey. Because it does not ferment lactose, expect colorless, translucent to slightly off-white colonies. The surrounding agar stays relatively clear without the pink haze or bile precipitate you see with strong lactose fermenters. Growth is typically rated good to excellent under standard conditions, so if you inoculate a healthy culture you should get visible colonies without much trouble.
The swarming behavior that makes Proteus species so recognizable on blood agar is largely suppressed on MacConkey. The bile salts and altered surface tension of the medium interfere with the flagella-driven motility that drives swarming. Some formulations go further: MacConkey variants marketed specifically for swarming control, such as MacConkey agar without salt, adjust composition to keep Proteus colonies discrete and readable. If you are using a standard MacConkey formulation and still see spreading, check whether your plates dried adequately before inoculation, since excess surface moisture can allow some motility even on inhibitory media.
Colony appearance summary
- Color: colorless to off-white or pale beige
- Transparency: translucent to slightly opaque
- Surrounding agar: clear, no pink haze
- Swarming: absent or greatly reduced compared to blood agar
- Size: moderate, discrete under standard formulations
How Proteus vulgaris compares to other Gram-negatives on the same plate
Putting Proteus vulgaris side by side with common Enterobacteriaceae on MacConkey makes interpretation much cleaner. The table below summarizes what you would typically see.
| Organism | Lactose fermentation | Colony color | Notable features |
|---|---|---|---|
| Proteus vulgaris | Non-fermenter | Colorless / off-white | Swarming suppressed; pale agar |
| Proteus mirabilis | Non-fermenter | Colorless / beige | Similar to P. vulgaris; same suppressed swarming |
| Escherichia coli | Strong fermenter | Pink to red | Often with bile precipitate; metallic sheen on EMB |
| Salmonella spp. | Non-fermenter | Colorless | No swarming; often needs further differentiation from Proteus |
| Klebsiella pneumoniae | Strong fermenter | Pink, mucoid | Mucoid texture distinguishes from E. coli |
| Pseudomonas aeruginosa | Non-fermenter | Colorless to pale | May produce pigment; distinctive odor |
The key practical point is that Proteus vulgaris falls into the non-lactose-fermenter (NLF) group along with Salmonella, Shigella, and Pseudomonas. Salmonella can grow on MacConkey agar, typically showing up as colorless, non-lactose-fermenting colonies that require further confirmation. Colorless colonies alone do not distinguish these organisms, so MacConkey gives you a starting point rather than a final ID. Follow-up biochemical tests, such as urease (strongly positive for Proteus), indole, and H2S production, are what separate Proteus vulgaris from the other NLF organisms on the plate. For comparison, Pseudomonas also shows up as a colorless non-fermenter on MacConkey, and Salmonella similarly produces pale colonies that require further testing to confirm identity. Pseudomonas is also expected to grow on MacConkey agar as a colorless, non-lactose-fermenting isolate.
Incubation conditions that matter
Standard incubation for MacConkey agar is 33 to 37°C for 18 to 24 hours in ambient air (aerobic). Some protocols extend this to 48 hours at 35°C plus or minus 2°C when working with slower growers, but for Proteus vulgaris, 18 to 24 hours is generally enough to get readable colonies. Avoid going much beyond 48 hours without a specific reason because extended incubation can complicate interpretation as colony morphology shifts and secondary metabolites accumulate.
Temperature accuracy is worth checking if your results look off. A thermometer inside the incubator rather than trusting the dial reading will tell you whether you are actually hitting the target range. Proteus vulgaris is not a fragile organism, but consistent temperature control is basic quality practice for any culture work.
Troubleshooting when growth is absent or unexpected
If you inoculate Proteus vulgaris on MacConkey and see no growth, or something that does not match expectations, work through these checks systematically. Does Burkholderia cepacia grow on MacConkey agar, and what colony appearance would you expect if it does?
- Check organism viability: was the culture stored and revived correctly? A stock that has been frozen and thawed improperly, or a subculture that has been sitting on the bench too long, may have lost viability. Run a parallel positive control on a non-selective medium like tryptic soy agar to confirm the organism is alive.
- Check inoculum density: too light an inoculation can fail to produce visible colonies within the standard read window. Use a 0.5 McFarland standard as a reference point if you need a consistent starting density.
- Verify media preparation: MacConkey agar should be autoclaved for 15 minutes at 121°C, then cooled to 45 to 50°C before pouring approximately 20 mL per plate. Overheating can degrade the neutral red indicator and alter bile salt activity. Underheating can leave the agar incompletely sterilized or poorly dissolved.
- Check media age and storage: plates stored past their expiration date, exposed to light, or left in non-ideal temperature conditions may show reduced performance. Neutral red is photosensitive; store plates in the dark.
- Confirm you are using standard MacConkey and not a variant: sorbitol-MacConkey, MacConkey without salt, or MacConkey No. 3 have different selectivity and performance characteristics. If you substituted a variant formulation, the expected results change. ATCC quality control warranties, for instance, specifically note that using an alternative medium formulation can void growth expectations.
- Look at colony color carefully under good lighting: very pale or translucent colorless colonies can be missed if plates are read against a dark background or in poor light. Holding the plate up to indirect light and examining the agar clarity around colonies can reveal growth that looked like nothing at first glance.
Unexpected pink or red colonies from a known Proteus vulgaris culture are a flag worth investigating. True pink colonies would suggest lactose fermentation, which is not characteristic of P. vulgaris. Double-check that you are working with the correct organism and that there is no contamination on the plate.
Biosafety and lab handling
Proteus vulgaris is handled at Biosafety Level 2 (BSL-2), which applies to agents that pose moderate risk and require standard precautions. In practice that means wearing a lab coat, gloves, and eye protection whenever you are handling cultures or inoculating plates. For procedures that could generate aerosols or splashes, work inside a biological safety cabinet.
Decontaminate all culture materials before disposal. Plates, tubes, and liquid waste should be autoclaved or treated with an approved chemical disinfectant before going into the waste stream. Microbiological cultures are regulated medical waste and on-site decontamination is the preferred approach. Do not leave spent MacConkey plates sitting open on the bench or in a regular trash bin.
Standard Precautions apply: treat every culture as potentially infectious, wash hands after removing gloves, and never eat, drink, or apply cosmetics in the lab. These habits matter especially in clinical or food safety labs where the identity of an isolate may not be fully confirmed before initial culture work begins.
What to do after you confirm growth
Once you have confirmed colorless, non-swarming growth on MacConkey and suspect Proteus vulgaris, the next step is confirmatory biochemical testing. Urease production is a strong positive for Proteus species, and the indole test helps separate P. vulgaris (indole positive) from P. mirabilis (indole negative). H2S production and motility tests round out the profile. MacConkey gives you the Gram-negative, non-lactose-fermenter starting point; the biochemical panel gives you the species.
If you are working in a food safety context, finding Proteus vulgaris in a food sample or environmental swab on MacConkey is an indication of fecal contamination or poor sanitation, since Proteus species are associated with decomposing organic matter and are not part of normal food flora. The MacConkey result flags it as a Gram-negative NLF organism; confirmatory ID and any required reporting follow from there.
FAQ
If Proteus vulgaris grows on MacConkey agar, why might my plate show no colonies or very scant growth?
Common causes are using an overly old culture, inoculating too lightly, or plates that are too dry or too wet. Also verify the incubation range (about 33 to 37°C) with an internal thermometer, since substantial temperature drift can delay or reduce colony formation.
Do all Proteus species produce the same colony appearance on MacConkey agar?
They’re usually colorless to off-white because they are non-lactose fermenters, but colony size and edge regularity can differ between species. The article’s main reliable cue for Proteus vulgaris is the lack of lactose fermentation plus confirmatory biochemistry, not exact colony morphology.
Will swarming ever happen on MacConkey plates with Proteus vulgaris?
It is often reduced, but it can still occur if surface conditions allow motility, especially with excess moisture or very wet plates. If you see spreading, recheck plate dryness and confirm you used the standard formulation intended for routine Gram-negative work.
How can I tell Proteus vulgaris apart from other non-lactose fermenters that also grow on MacConkey?
Use confirmatory tests rather than colony color alone. For example, Proteus vulgaris is urease strongly positive, and indole helps separate it from close relatives like P. mirabilis (indole negative), while Salmonella and Pseudomonas require their own biochemical and selective follow-up.
If I see pink colonies on a plate inoculated with Proteus vulgaris, does that mean the organism changed?
Not usually. True lactose fermentation would suggest a different organism or contamination, because P. vulgaris is not expected to ferment lactose. Double-check strain identity (source and labeling), then examine whether the pink colonies match the colony type from the inoculum.
Can MacConkey results distinguish whether the isolate is Gram-negative without a Gram stain?
MacConkey is selective for Gram-negative growth, so Gram-positive bacteria are generally inhibited. However, you still cannot confirm Gram reaction solely from colony color, so if the result is unexpected you should perform a Gram stain as part of verification.
How long should I incubate MacConkey agar for Proteus vulgaris before deciding the test is negative?
Typical readability is achieved within 18 to 24 hours at 33 to 37°C. If you still see no growth after the standard window, that supports a true negative more than a delayed positive, but if your workflow uses extended incubation, avoid going far beyond 48 hours without a reason because colony appearance can change over time.
Is MacConkey agar suitable for presumptive screening in food or environmental samples, and what’s the next step?
It works as a presumptive screen for Gram-negative non-lactose fermenters, including Proteus-like profiles. The next step is confirmatory identification with biochemical testing before any reporting or regulatory actions, since multiple organisms can produce colorless colonies on the same medium.
Citations
FDA BAM Media M91 lists MacConkey agar components per 1 L including lactose (10 g), bile salts (e.g., 1.5 g), neutral red (0.03 g), and crystal violet (0.001 g), driving both selectivity and lactose-differentiated color.
BAM Media M91: MacConkey Agar | FDA - https://www.fda.gov/food/laboratory-methods-food/bam-media-m91-macconkey-agar
FDA BAM Media M91 provides preparation instructions: autoclave 15 min at 121°C, cool to 45–50°C, and pour 20 mL into sterile Petri dishes—relevant for troubleshooting poor growth if preparation/temperature control is off.
BAM Media M91: MacConkey Agar | FDA - https://www.fda.gov/food/laboratory-methods-food/bam-media-m91-macconkey-agar
StatPearls describes MacConkey agar as a selective and differential medium where lactose fermenters form pink colonies and non-lactose fermenters form off-white/opaque colonies.
MacConkey Medium - StatPearls - NCBI Bookshelf - https://www.ncbi.nlm.nih.gov/books/NBK557394/
Liofilchem’s IFU states lactose fermentation decreases pH via organic acids, and that lactose fermenters form pink colonies while non-lactose fermenters form off-white (implying neutral red color change is tied to lactose fermentation).
Liofilchem® - MacConkey Agar - Rev.0 / 12.03.2024 (IFU) - https://www.liofilchem.net/login/pd/ifu/10029_IFU.pdf
Remel/ Thermo Fisher IFU: incubate MacConkey agar plates 33–37°C for 18–24 hours in ambient air; lactose fermenters are pink and non-lactose fermenters form colorless colonies.
Thermo Fisher Scientific Remel MacConkey Agar (IFU1550) - https://assets.thermofisher.com/TFS-Assets/LSG/manuals/IFU1550.pdf
Thermo Fisher MacConkey No.3 specification lists incubation of 24–48 hours at 35°C ±2°C (aerobic) and expected results: E. coli gives pink colonies; Proteus mirabilis gives colorless/beige colonies.
TECHNICAL SPECIFICATION (Thermo Fisher) - MacConkey No.3 agar (PP2130 2022/03) - https://documents.thermofisher.com/TFS-Assets/MBD/Specification-Sheets/MacConkey%20No3%20PP2130%202022%2003.pdf
ASM JCM paper notes that dryness conditions and specific MacConkey-related factors (including bile salts) are among conditions/medium components that can abolish or reduce Proteus swarming.
Abolition of Swarming of Proteus (Journal of Clinical Microbiology, ASM) - https://journals.asm.org/doi/10.1128/jcm.37.10.3435-3435.1999
TMCC notes that organisms not fermenting lactose remain colorless/translucent and gives “good to excellent, colorless colonies without bile precipitate” as indicative for Proteus vulgaris (among examples).
Truckee Meadows Community College - Culture Media (MacConkey agar) - https://www.tmcc.edu/microbiology-resource-center/culture-media
Remel IFU1550 states swarming of Proteus can be affected by MacConkey medium composition: it describes that the modification of MacConkey resulting has improved inhibition of swarming by Proteus spp.
Thermo Fisher (Remel) IFU1550 MacConkey Agar - https://assets.thermofisher.com/TFS-Assets/LSG/manuals/IFU1550.pdf
In a discussion of Proteus on MacConkey, the excerpt notes that P. mirabilis does not ferment lactose and remains colorless on MacConkey; it also states bile salts in MacConkey are involved in inhibiting swarming.
Proteus mirabilis: Methods and Protocols (book excerpt via dokumen.pub) - https://dokumen.pub/proteus-mirabilis-methods-and-protocols-1st-ed-978-1-4939-9600-1978-1-4939-9601-8.html
ATCC highlights that if an alternative medium formulation/reagent is used, warranty for viability may no longer apply—relevant to troubleshooting “no growth” when media composition differs from QC expectations.
ATCC 29906 Proteus mirabilis product page - https://www.atcc.org/products/29906
ASM protocol materials emphasize that gram-negative bacteria on MacConkey appear lactose-fermenter vs non-fermenter (pink vs colorless) and discuss differentiation based on lactose fermentation intensity.
MacConkey Agar Plates Protocols (ASM PDF) - https://asm.org/ASM/media/Protocol-Images/MacConkey-Agar-Plates-Protocols.pdf
Thermo Fisher expected results explicitly include P. mirabilis: “4-5+ growth, colourless/beige colonies,” providing a concrete Proteus vs lactose-fermenter comparison on MacConkey No.3.
Thermo Fisher Scientific Technical Specification - MacConkey No.3 agar (PP2130) - https://documents.thermofisher.com/TFS-Assets/MBD/Specification-Sheets/MacConkey%20No3%20PP2130%202022%2003.pdf
LibreTexts describes that if bacteria do not ferment lactose, colonies/confluent growth are colorless and the surrounding agar remains relatively transparent; an example image shows Proteus mirabilis on MacConkey with no lactose fermentation.
Biology LibreTexts - 12.6.1 MacConkey Agar results - https://bio.libretexts.org/Learning_Objects/Laboratory_Experiments/Microbiology_Labs/Microbiology_Labs_II/12%3A_Isolation_and_Identification_of_Enterobacteriaceae_and_Pseudomonas_Part_1/12.06%3A_Results_from_Testing_Procedures_Used_in_This_Lab/12.6.01%3A_MacConkey_Agar
Thermo Fisher IFU1550 lists non-lactose-fermenters (e.g., Salmonella and Shigella) as colorless on MacConkey, which helps distinguish Proteus-style NLF colonies from lactose fermenters on the same plate.
Thermo Fisher Scientific - MacConkey Agar (IFU1550) - https://assets.thermofisher.com/TFS-Assets/LSG/manuals/IFU1550.pdf
StatPearls notes that MacConkey’s differentiation depends on lactose fermentation rate and also that colony appearance can vary with species traits (e.g., capsule mucoid colony forming species like Klebsiella/Enterobacter).
StatPearls - MacConkey Medium (NCBI Bookshelf) - https://www.ncbi.nlm.nih.gov/books/NBK557394/
LibreTexts explains the reagent logic: crystal violet and bile salts inhibit Gram-positive bacteria; neutral red stains microbes fermenting lactose (via pH drop) resulting in pink/red colonies.
Biology LibreTexts - 22.2 Selective and Differential Media (MacConkey/EMB/MSA) - https://bio.libretexts.org/Courses/College_of_the_Canyons/Bio_221Lab%3A_Introduction_to_Microbiology_%28Burke%29/22%3A_Physiological_Tests_for_Characterization_and_Identification_of_Bacteria/22.02%3A_Selective_and_Differential_Media_-_MacConkey_EMB_MSA
CDC training page: BSL-2 includes use of a biological safety cabinet (BSC) or other containment for procedures that can cause infection from aerosols or splashes, and includes decontamination (e.g., autoclave or alternative) for proper disposal.
CDC LC Quick Learn: Recognize the four Biosafety Levels (CDC) - https://www.cdc.gov/training/QuickLearns/biosafety/index.html
CDC MMWR guidance: because the identity of infectious agent may be unknown in clinical labs, it generally recommends following BSL-2 standard and special practices and OSHA Standard Precautions (gloves, gown, protective eyewear) for handling blood/body fluids and potentially infectious agents.
CDC - Guidelines for Safe Work Practices in Human and Animal Medical Diagnostic Laboratories (MMWR) - https://www.cdc.gov/mmwr/preview/mmwrhtml/su6101a1.htm
CDC infection-control guidance: microbiologic cultures/stocks are regulated medical waste and should be decontaminated before disposal; CDC notes on-site decontamination is preferred for BSL-2-type laboratories.
CDC - Regulated Medical Waste (infection control) - https://www.cdc.gov/infection-control/hcp/environmental-control/regulated-medical-waste.html
VUMC summarizes that BSL-2 is suitable for work with moderate-risk agents and typically includes lab coats, gloves, and appropriate containment practices; it also emphasizes decontaminating waste before disposal.
Office of Clinical and Research Safety (VUMC) - Basics of Biosafety Level 2 - https://www.vumc.org/safety/bio/basics-biosafety-level-2
HHS ASPR biosafety guidance: PPE is worn, and BSL-2 labs must have access to equipment to decontaminate laboratory waste (e.g., autoclave and/or other decontamination method) depending on risk assessment.
ASPR HHS - Biosafety Level Requirements - https://aspr.hhs.gov/S3/Pages/Biosafety-Level-Requirements.aspx
Thermo Fisher expects Proteus mirabilis on MacConkey No.3 to show colorless/beige colonies (non-lactose fermenter pattern) under the specified incubation (24–48h at 35°C ±2°C).
Thermo Fisher Technical Specification (MacConkey No.3) - incubation/expected results - https://documents.thermofisher.com/TFS-Assets/MBD/Specification-Sheets/MacConkey%20No3%20PP2130%202022%2003.pdf
Thermo Fisher IFU1550 advises not to incubate without regard to time/conditions; prolonged incubation may complicate interpretation (noted in related IFU guidance pages) and supports using the stated 18–24 hour range at 33–37°C for standard reading.
Thermo Fisher - IFU1550 MacConkey Agar - https://assets.thermofisher.com/TFS-Assets/LSG/manuals/IFU1550.pdf
Thermo Fisher notes common autoclave/media preparation issues: incorrect autoclave time for media volume/type can lead to problems (e.g., overcooking/damaging media or inadequate processing), which can contribute to unexpected growth/no growth on plates.
Thermo Fisher - Media preparation errors: autoclave problems (Microbiology Learning Center) - https://www.thermofisher.com/nz/en/home/industrial/microbiology/microbiology-learning-center/media-preparation-errors-autoclave-problems.html
UNR biosafety manual emphasizes decontamination as a core lab safety step to protect personnel and prevent release of infectious organisms; it can be used to justify autoclaving/approved disinfection steps for cultures and contaminated plate waste.
University of Nevada, Reno - Decontamination (biosafety manual chapter) - https://www.unr.edu/ehs/policies-manuals/biosafety-manual/chapter-7
Thermo Fisher’s “MacConkey agar without salt” guidance documents changes in expected performance/Proteus appearance (quality control adjusts for swarming control), illustrating that MacConkey variants can alter Proteus morphology/swarm visibility and thus interpretation.
Thermo Fisher - MacConkey Agar without salt (Controlled swarming concept) IFU-0445 - https://documents.thermofisher.com/TFS-Assets/MBD/Instructions/MBD_BT_IFU-0445%20CM0507B%20MAC-CONKEY%20AGAR%20%28WITHOUT%20SALT%29.pdf
Sorbitol-MacConkey agar is a variant used to distinguish organisms based on sorbitol fermentation (e.g., for O157:H7), meaning colony color interpretation depends on the specific sugar substrate in the variant rather than lactose alone.
Sorbitol-MacConkey agar (Wikipedia) - https://en.wikipedia.org/wiki/Sorbitol-MacConkey_agar
ASM protocols indicate that colonies on MacConkey are interpreted primarily by lactose fermentation (pink/red vs colorless) and by colony morphology cues, which is essential when Proteus growth is ambiguous or atypical.
ASM MacConkey Agar Plates Protocols (ASM PDF) - https://asm.org/ASM/media/Protocol-Images/MacConkey-Agar-Plates-Protocols.pdf
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