PALCAM, Oxford and Chromogenic Agars for Listeria: A Scientific Comparison

Post Date:
02Nov, 2026

Author

Ms. Prima Gonsalves

Designation

Deputy R&D Manager- Microbiology, HiMedia Laboratories Pvt. Ltd.

Introduction

The detection of Listeria monocytogenes in food and environmental samples is an important part of microbiological quality and food safety testing. Selective agar media help laboratories recover presumptive Listeria colonies while suppressing competing microorganisms. However, not all selective media distinguish L. monocytogenes from other Listeria species.

Oxford agar, modified Oxford agar (MOX) and PALCAM agar rely on esculin hydrolysis, which produces characteristic blackening around growing colonies. Chromogenic media, such as HiMedia® HiCrome® Listeria formulations, use enzyme-substrate reactions to provide additional differentiation, including a characteristic colony color and, in applicable formulations, an opaque halo associated with L. monocytogenes.

Understanding these differences helps laboratories interpret plates, select appropriate media and follow the requirements of their testing method. The FDA Bacteriological Analytical Manual (BAM), Chapter 10, specifies plating onto one esculin-based selective agar and one chromogenic selective agar during its isolation procedure.

Comparison of Listeria selective and chromogenic agars

The table below summarizes typical colony appearance, practical strengths, limitations and corresponding HiMedia® products. Actual results may vary with the strain, incubation conditions and medium formulation.

Table 1. Comparison of Selective and Chromogenic Listeria Agars and Corresponding HiMedia® Products

Medium Typical presumptive Listeria appearance Main strength Key limitation Relevant HiMedia® product
Oxford agar Gray to black colonies with a black halo; colonies may develop a sunken center with longer incubation Supports presumptive detection of Listeria spp. Does not reliably differentiate L. monocytogenes from other Listeria species *Listeria *Oxford Medium Base w/ 1.2% Agar (M1145F), with the specified supplement, with the specified supplement like CM Selective Supplement Modified (FD126)
Modified Oxford agar (MOX) Black colonies with a black halo Selective isolation in relevant food-testing methods Species-level differentiation requires additional testing Modified Listeria Oxford Agar Base (M1897), ColCef of selective supplement (FD306)
PALCAM agar Gray-green colonies with a black sunken center and black halo Mannitol fermentation helps distinguish certain competing organisms Colony appearance alone does not identify L. monocytogenes PALCAM Agar Base (M1064) with PALCAM Selective Supplement (FD061)
Chromogenic Listeria Agar (ALOA Type) – Agar Listeria according to Ottaviani and Agosti Blue-green colonies; L. monocytogenes typically develops an opaque halo Helps differentiate presumptive L. monocytogenes from other Listeria species on the same plate Some strains may develop a halo slowly; other species, including L. ivanovii, may also produce halos HiCrome® Listeria Ottaviani Agosti Agar Base (M1540I), L.mono Enrichment Supplement I (FD214) and OA selective supplement (FD 212A)
Modified chromogenic Listeria agar Appearance depends on the formulation and the target organism's reactions Provides an additional chromogenic approach for presumptive differentiation Interpret results using the product-specific technical data sheet and validated method HiCrome® Listeria Oxford Agar Base (M1145F), CA Selective supplement (FD181)

Important: Confirm the required medium, formulation, supplement and incubation conditions against the current method and the relevant product documentation. Do not assume that different commercial formulations are interchangeable solely because they have similar names.

How Oxford and PALCAM agar work?

Oxford and PALCAM are selective and differential media used for the presumptive isolation of Listeria. Both contain esculin and a suitable iron salt. When Listeria hydrolyzes esculin, the resulting esculetin reacts with iron to produce a dark complex. This causes blackening of the colonies or surrounding medium.

Selective components, including lithium chloride and specified antimicrobial agents, help suppress competing bacteria. Nevertheless, background organisms may still grow, and not every dark colony necessarily represents Listeria.

Oxford agar: recognizing black colonies and halos.

On Oxford agar, typical Listeria colonies are approximately 1 mm in diameter and gray to black, with a black halo after 24 hours at 35°C. After 48 hours, they may become 2–3 mm in diameter, with a darker appearance and sunken center. These are the typical characteristics described in FDA BAM Chapter 10.

For laboratories using Oxford agar, HiCrome® Listeria Oxford Agar Base w/ 1.2% Agar (M1145F) is an available product option. Prepare the complete medium with the appropriate selective supplement according to the manufacturer's instructions and the applicable method.

Modified Oxford agar: a selective alternative.

Modified Oxford agar uses a related esculin-based principle but has a modified selective formulation. It supports presumptive Listeria isolation in relevant food-testing procedures.

Typical colonies show blackening and a dark halo. As with standard Oxford agar, these features do not establish the species identity of the isolate.

HiMedia® Modified Listeria Oxford Agar Base (M1897) is relevant for laboratories evaluating MOX-based workflows. The correct supplement and preparation procedure should be selected according to the applicable method and product instructions.

PALCAM agar: additional differentiation through mannitol fermentation.

PALCAM agar combines esculin hydrolysis with mannitol fermentation. Typical Listeria colonies appear gray-green, with a black center and surrounding dark halo against the red background of the medium.

Mannitol and the pH indicator phenol red provide an additional differentiation mechanism. Certain competing organisms that ferment mannitol produce yellow colonies or yellowing of the surrounding medium, helping analysts distinguish them from typical presumptive Listeria colonies.

PALCAM therefore offers more than a blackening reaction alone, but it does not reliably differentiate L. monocytogenes from all other Listeria species.

For laboratories using this medium, HiMedia® offers PALCAM Agar Base (M1064) and PALCAM Selective Supplement (FD061). Use the specified supplement and preparation instructions for the selected procedure. The FDA also provides a reference formulation for PALCAM selective agar.

Chromogenic Listeria agar: differentiating presumptive *L. monocytogenes*.

Chromogenic media use enzyme-substrate reactions to help differentiate microorganisms by colony color. In ALOA-type formulations, a chromogenic substrate associated with β-glucosidase activity produces blue-green colonies for Listeria species. A second reaction associated with phospholipase activity produces an opaque halo around typical L. monocytogenes colonies.

This combination helps laboratories distinguish presumptive L. monocytogenes from other Listeria species directly on the plate. However, halo formation is not exclusive to L. monocytogenes: L. ivanovii may also produce a halo, and some strains may develop characteristic reactions more slowly.

The other combination of chromogenic media is based on β-glucosidase and xylose fermentation.

HiMedia® offers HiCrome® Listeria Ottaviani Agosti Agar Base (M1540I) and HiCrome Listeria Agar Base, Modified (M1417). Since chromogenic reactions depend on the specific formulation, laboratories should use the relevant HiMedia® technical data sheet to interpret colony appearance rather than assume that every formulation produces identical results.

Table 2. Comparison of Listeria Agar Media Based on Differentiation Principles, Colony Appearance, and Diagnostic Applications

Comparison point Oxford / MOX PALCAM Chromogenic Listeria agar
Primary differentiation principle Esculin hydrolysis and iron reaction Esculin hydrolysis plus mannitol fermentation Enzyme-substrate reactions
Typical visual appearance Black colonies or black halo Gray-green colonies with black centers against a red background a) Blue-green colonies; an opaque halo may indicate presumptive L. monocytogenes in ALOA-type media
b) Blue-green colonies without a yellow halo may indicate presumptive Listeria monocytogenes on certain chromogenic Listeria media, depending on the medium formulation. Confirmatory testing is required for accurate species identification.
Differentiates L. monocytogenes on the plate No No Provides presumptive differentiation, depending on the formulation
Main interpretation concern Other organisms may produce confusing growth or reactions Mannitol-fermenting contaminants and other look-alikes may complicate interpretation Halo reactions are not completely species-specific; atypical strains may occur
Role in testing Selective isolation of presumptive Listeria Selective isolation with an additional differential reaction Additional differentiation of presumptive L. monocytogenes

What do FDA BAM and USDA-FSIS methods specify?

The applicable regulatory or reference method determines which media, and incubation conditions should be used.

FDA BAM Chapter 10: The isolation procedure calls for streaking enrichment cultures onto one esculin-based selective agar and one chromogenic selective agar at the specified sampling points, including 24 and 48 hours. The chapter lists Oxford, PALCAM, modified Oxford and other suitable esculin-based options, alongside chromogenic options. Follow the current chapter for exact preparation, incubation and interpretation requirements.

USDA-FSIS MLG 8.15: This method addresses isolation and identification of L. monocytogenes and Listeria spp. from specified meat, poultry, egg-product and environmental samples. Laboratories should consult the current MLG document for the prescribed media, incubation conditions and colony-selection criteria rather than assuming the FDA BAM procedure is identical.

A commercial medium should only be used in a regulated workflow when its formulation and preparation comply with the applicable method requirements.

Conclusion

Oxford, modified Oxford and PALCAM agars, support the selective isolation of presumptive Listeria, while chromogenic media provide additional differentiation of presumptive L. monocytogenes. The key distinction is the detection principle: esculin hydrolysis produces blackening on traditional media, whereas chromogenic reactions provide characteristic colony colors and, in appropriate formulations, an opaque halo.

HiMedia offers options across these categories, including Listeria Oxford Agar Base w/ 1.2% Agar (M1145F), Modified Oxford Agar Base (M1897), PALCAM Agar Base (M1064) and HiCrome® Listeria Ottaviani Agosti Agar Base (M1540I),HiCrome® Listeria Agar Base (M1417F) Selecting the appropriate medium depends on the sample, target organism, applicable method and product-specific preparation requirements. Presumptive colony appearance should always be followed by the confirmation steps required by the laboratory's validated procedure.

Acknowledgement

The authors express their sincere gratitude to Dr. Rahul Warke (Director, Microbiology Department) and Dr. Girish Mahajan (Senior Vice President, Microbiology Department) for their exceptional scientific guidance, critical insights, and continuous support throughout the development of this work. The authors also thank Ms. Vrutti Mistry (Scientific Writer) for her support in drafting the manuscript.

References

  1. U.S. Food and Drug Administration (FDA). Bacteriological Analytical Manual (BAM), Chapter 10: Detection of Listeria monocytogenes in Foods and Environmental Samples, and Enumeration of Listeria monocytogenes in Foods.
    https://www.fda.gov/food/laboratory-methods-food/bam-chapter-10-detection-listeria-monocytogenes-foods-and-environmental-samples-and-enumeration
  2. U.S. Food and Drug Administration (FDA). Bacteriological Analytical Manual (BAM), Media M118a: PALCAM Listeria Selective Agar.
    https://www.fda.gov/food/laboratory-methods-food/bam-media-m118a-palcam-listeria-selective-agar
  3. U.S. Department of Agriculture, Food Safety and Inspection Service (USDA-FSIS). Microbiology Laboratory Guidebook (MLG), Chapter 8.15: Isolation and Identification of Listeria monocytogenes and Listeria spp. from Ready-to-Eat Meat, Poultry, Siluriformes (Catfish), Egg Products, and Environmental Samples.
    https://www.fsis.usda.gov/sites/default/files/media_file/documents/MLG-8.15.pdf

Frequently Asked Questions

Find answers to commonly asked questions about this topic.

What do Listeria colonies look like on PALCAM agar?
Typical colonies are gray-green with a black sunken center and black halo. Mannitol-fermenting contaminants may produce yellow reactions, which can aid interpretation.
What do Listeria colonies look like on Oxford agar?
Typical colonies are gray to black with a black halo at 24 hours. By 48 hours, they may become larger, darker and sunken in the center under the conditions specified by FDA BAM.
What is the main difference between PALCAM and Oxford agar?
Both rely on esculin hydrolysis to produce dark colonies or halos. PALCAM additionally uses mannitol fermentation and a pH indicator to help differentiate certain competing organisms. Neither medium alone confirms L. monocytogenes.
Why are colonies blue-green on chromogenic Listeria agar?
In applicable formulations, β-glucosidase activity produces the blue-green chromogenic reaction. An additional phospholipase-associated reaction may produce an opaque halo around presumptive L. monocytogenes colonies.
Can chromogenic agar replace Oxford or PALCAM agar?
Not automatically. FDA BAM Chapter 10 specifies the use of both an esculin-based agar and a chromogenic agar in its isolation procedure. Follow the relevant method and laboratory validation requirements.