About 45 years ago, the first solid chromogenic media was invented and patented in France. The idea was born out of the thought that it might be advantageous to be able to identify various bacterial types on solid agar by the color of colonies. The name comes from the Greek word chroma, meaning color.

The science, as noted in a 2020 article published on the American Society for Microbiology page, is described in this way:

1. The media contains molecules called chromogens. A chromogen molecule consists of a carbohydrate substrate (the ‘key’ to a specific enzyme ‘lock’), as well as a chromophore. The chromogen is colorless because the chromophore does not absorb visible light while conjugated to the substrate.
2. When a bacterial organism with specific enzymatic activity comes into contact with the chromogen molecule, that enzyme cleaves the chromogen molecule, releasing the chromophore.
3. When the chromophore is not conjugated, its color becomes visible. By using a chromophore that does not diffuse readily into the surrounding media, the color stays concentrated in the area where the bacterial colony with the target enzymatic activity grew. Thus, the colony itself takes on the chromophore's color (the color is the reflected wavelength of light). (1)

In its solid media state, the evolution of chromogenic media has transformed the way microbiology laboratories in all segments, (clinical, food, environmental), are able to quickly identify different species of bacteria and yeast by the naked eye.

The advantages of chromogenic media include increased specificity, reduced time for identification, reduced cost, and improved ease of use. It has been particularly valuable in clinical microbiology laboratories for accurate identification of pathogenic bacteria. Due to its high specificity, in many cases follow-up confirmation tests are not needed.

As with other methods, chromogenic media will rarely give false positives from organisms that are showing similar biochemical reactions. The average time-to-result for the chromogenic media is from 24 to 48 hours. While this makes it faster than traditional methods, it is still slower than some other alternative methods like polymerase chain reaction (PCR) and immunologic methods. Another drawback is the need for incubator space.

Specific to food, and because chromogenic media is considered an alternative method, it needs to be validated following IS0 16140-1&2 and by the customer following ISO 16140-3.

Advantages of Chromogenic Media

The benefits of using chromogenic media include faster results, reliable visual detection and additional testing, such as susceptibility testing, are possible directly from isolated colonies on the media. Compared with the use of conventional culture media, this often results in cost savings from reduced labor time and reduced use of reagents as fewer biochemical and/or serological confirmation tests are required.

Chromogenic media are available as a dehydrated bulk powder or ready-to-use formats and can be utilized as presence/absence tests.

In summary, benefits of chromogenic media include:

• Enhanced accuracy
• Easy microbial detection and identification using color
• Cost-efficient working process
• Faster bacterial identification and results
• Available as dehydrated media or in ready-to-use formats
• Only requires basic microbiology skills to use

Hardy Diagnostics was the first company to introduce chromogenic media to the United States in 1996. Hardy manufactures and sells its chromogenic media line called HardyCHROM™ across the globe with more than 20 chromogenic agars currently commercially available. See the complete line up HERE.

The latest two releases include HardyCHROM™ Group A Strep Chromogenic Agar (Cat. no. G337) and HardyCHROM™ Candida + auris (Cat. no. G343) Chromogenic Agar, which not only displays bright teal colors to the naked eye, but positive colonies of Candida auris will also fluoresce under UV light.

Hardy continues to innovate and expand its catalog of chromogenic agars with improved specificity and a greater range of targeted organisms.

If you’ve ever spent time in the woods or forests during the spring and summer months, checking for ticks was probably one of the first things you did when you returned home or got back to camp. This is an important habit because a tick you picked up may be carrying one of the bacteria associated with Lyme disease, mainly Borrelia burgdorferi or rarely, Borrelia mayonii.(1) 

Lyme disease affects nearly half a million people in the United States annually. (2) Lyme disease can be a particularly debilitating and long lasting disease. Symptoms can include fever, headache, fatigue, and the distinctive “bulls-eye” rash associated with the site of the tick bite. Though the infection in most cases is cleared through antibiotic use, symptoms can linger for months or even years after. (2) If left untreated, the infection can spread to joints, the heart and eventually the nervous system. (1) 

Recently, researchers at MIT and the University of Helsinki have discovered that a protein in human sweat can protect against Lyme disease. The mechanism in which this protein is able to inhibit the growth of these bacteria is still unknown. Researchers hope to use this protein to create topical creams that could prevent or possibly treat infections that don’t respond to antibiotics. (2) 

Researchers at Stanford University ran a Genome-Wide Association Study (GWAS) on a data-set of 410,000 people, 7,000 of which were diagnosed with Lyme disease. This analysis yielded a hit on a particular genetic variant of a secretoglobin called SCGB1D2(2)  Secretoglobins are a family of proteins found in lungs and other organs that play a role in immune response to infections. (2)  This particular secretoglobin is 1 of 11 produced by the human body. Only about 1/3 of the human population carries this particular genetic variant naturally, which appears to significantly decrease Borrelia burgdorferi growth. (2) 

Researchers tested a ‘normal’ version of SCGB1D2 and then created a mutated version to test on mice. The mice that had been exposed to the mutated version of the protein developed Lyme disease when they were exposed to Borrelia burgdorferi. The mice that had been exposed to the ‘normal’ version of SCGB1D2 did not develop an infection. (2)  

Researchers did mention that just because a person may carry this particular variant of SCGB1D2, they should not assume they are immune to developing Lyme disease. A major factor in this protein preventing infection is if the individual is actively sweating, or has been sweating on the site of a tick bite. (2)  

Researchers hope to continue studying secretoglobins with the hope that they could be a class of antimicrobial proteins whose potential we have not yet fully realized. (2) For a disease that we still have much to learn about, these developments in treatment for Lyme disease are exciting for immunologists and outdoorsmen alike. 

By Joe Tripp, Technical Support Microbiologist

Sources Cited: 1, 2

 As spring cleaning approaches, it could be a good idea to think about our disinfectant selections as it has been suggested that using disinfectants in households and healthcare facilities may increase the risk of antibiotic resistance.

A research team at ARC Center of Excellence in Synthetic Biology led by Dr. Liping Li in Australia, has been investigating the potential emergence of antimicrobial-resistant bacteria due to biocides.(1) The study group examined the consequences of applying biocides at various concentrations. Eight out of ten biocides tested at low concentrations “de-energized” the bacterial cell membrane. In light of this, the current understanding is that low concentration biocides and surface residues are antagonizing the antibiotics, compromising their potency and making it difficult for them to enter the cell to reach their intended targets, thus promoting the “superbug” characteristics of these highly drug resistant microorganisms.

For instance, the multi-drug resistant Acinetobacter baumannii is on the World Health Organization's "most wanted" list, and the race is on to find a way to vanquish it (4). One of the most feared human pathogens, A. baumannii especially threatens immune-compromised patients in hospital intensive care units. It's one of the so-called ESKAPE pathogens, which includes Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Pseudomonas aeruginosa, and Enterobacter spp.

It is still not clear, says Dr. Li, why superbugs resist hospital biocides because in theory, the biocide concentrations used during disinfection are high enough to kill microbes.

In addition, it is suggested that persistent residues of common biocides, such as, chlorhexidine, benzalkonium, CTAB (cetyltrimethylammonium bromide), and others that are hard to remove from the environment—could induce antibiotic tolerance and the development of antimicrobial resistance.

The study emphasizes the need for more awareness and caution in the use of disinfectants in both the healthcare and household settings.(1) While these products play a crucial role in maintaining hygiene and preventing the spread of infectious diseases, they may be causing unintended consequences. Consumers must be educated about proper disinfectant usage, including dilution ratios, contact times, and targeted application areas. Following guidelines, such as those provided by the Centers for Disease Control and Prevention (CDC), can help determine when and how to clean, sanitize, and disinfect our homes.(2, 3)

Dr. Li concluded, "The findings in our study suggest that residual biocides could compromise antibiotic potency, at least in the lab; but we cannot predict whether this is true in a real-life scenario, because this needs further investigation. We hope our study will raise awareness of using biocides wisely and promote more biocide-related research."

By addressing the potential risks associated with disinfectants and implementing proactive measures to mitigate them, we can protect both public health and the efficacy of antimicrobial treatments for generations to come.



Elide Herrera

Technical Services Microbiologist II

References: 1, 2, 3, 4

Health officials in Japan racing to identify the reason for an alarming rise in streptococcal toxic shock syndrome, which has a 30% fatality rate.

The number of cases in 2024 is expected to exceed last year’s record numbers, while concern is growing that the harshest and potentially deadliest form of group A streptococcal disease – streptococcal toxic shock syndrome (STSS) – will continue to spread, after the presence of highly virulent and infectious strains were confirmed in Japan.

The National Institute of Infectious Diseases (NIID) said: “There are still many unknown factors regarding the mechanisms behind fulminant (severe and sudden) forms of streptococcus, and we are not at the stage where we can explain them.”

Provisional figures released by the NIID recorded 941 cases of STSS were reported last year. In the first two months of 2024, 378 cases have already been recorded, with infections identified in all but two of Japan’s 47 prefectures.

While older people are considered at greater risk, the Group A strain is leading to more deaths among patients under 50, according to NIID. Of the 65 people under 50 who were diagnosed with STSS between July and December in 2023, about a third, or 21, died, the Asahi Shimbun newspaper reported.

Most cases of STSS are caused by Streptococcus pyogenes. More commonly known as Strep A – it can cause sore throats, mainly in children, even though many people have it as colonizers and do not become ill.

But the highly contagious bacterial strain that causes this infection can, in some cases, cause serious illnesses, health complications and death, particularly in adults over 30. About 30% of STSS cases are fatal.

Older people can experience cold-like symptoms but in rare cases, the symptoms can worsen to include strep throat, tonsillitis, pneumonia and meningitis. The infection can come on quickly and symptoms can include a high temperature, muscle aches, a raised skin rash that feels like sandpaper and flu-like symptoms. In the most serious cases it can lead to organ failure and necrosis.

Ken Kikuchi, a professor of infectious diseases at Tokyo Women’s Medical University, says he is “very concerned” about the dramatic rise this year in the number of patients with severe invasive streptococcal infections.

“In my opinion, over 50% Japanese people have been infected by Sars-CoV-2 [the virus that causes Covid-19],” Kikuchi tells the Guardian. “People’s immunological status after recovering from Covid-19 might alter their susceptibility to some microorganisms. We need to clarify the infection cycle of severe invasive Streptococcal diseases and get them under control immediately.” Covid has been known to weaken the immune system and it is thought to be responsible for the rise in many other diseases recently.

Streptococcal infections, like those of Covid-19, are spread through droplets and physical contact. The bacterium can also infect patients through wounds on the hands and feet.

Strep A infections are treated with antibiotics, but patients with the more severe invasive Group A streptococcal disease are likely to need a combination of antibiotics and other drugs, along with intensive medical attention.

References: 1, 2, 3