Definition/Introduction

MacConkey agar (MAC) is a bacterial culture medium named after bacteriologist Alfred T. MacConkey (1861-1931). MacConkey agar is a selective and differentiating agar that only grows gram-negative bacterial species; it can further differentiate the gram-negative organisms based on their lactose metabolism.[1] The selective and differentiating properties of MacConkey agar enables utilization for both research and clinical applications. The fermentation of lactose produces organic acids, particularly lactic acid, which decreases the pH of the agar. MAC contains a pH indicator that turns pink under acidic conditions. Therefore, lactose-fermenting-gram-negatives (lactose-fermenters) will form pink colonies, while non-lactose fermenters will form off-white opaque colonies. Even within lactose-fermenters, species will show a varying rate of growth. The rate of growth is also a way to further differentiate organisms in the MAC medium. Lastly, some species that forms a capsule appear differently. Altogether, MacConkey agar only grows gram-negative bacteria, and those bacteria will appear differently based on their lactose fermenting ability as well as the rate of fermentation and the presence of a capsule or not. This makes MAC a powerful tool in differentiating and isolating bacterial species from the sample source. MAC is one of the many bacterial cultures clinical microbiologists utilize for diagnostic testing. It is still widely used in the clinical laboratory to identify causal agents from a patient (i.e., stool sample).

Issues of Concern

Key components of the MacConkey medium include crystal violet dye, bile salts, lactose, and neutral red (pH indicator). Crystal violet dye and bile salts halt the growth of gram-positive bacteria. This allows only gram-negative species to form colonies on MAC agar.[2] MacConkey agar contains the essential nutrients required for microorganism growth. Additional key components include crystal violet dye, bile salts, lactose, and neutral red (a pH indicator). The lactose in the agar is a source of fermentation. Lactose-fermenting microorganisms will produce organic acids, particularly lactic acid, which will lower the pH. Neutral red is a pH indicator that turns from off-white to bright red/pink as the pH drops below 6.8.

Based on the ability to ferment lactose, different species will yield colonies in varying appearance on a MacConkey medium. This gives McConkey agar its differentiating property. 

• Lactose (Lac) positive (pink colonies):
• Lactose fermenting species will grow pink colonies. Lactose fermentation will produce acidic byproducts that lower the pH, and this turns the pH indicator to pink.
• Example of Lac positive species: Escherichia coli, Enterobacteria, Klebsiella 
• Lac negative (white colonies)
• Gram-negative bacterial species will still form colonies, but colonies will have a white appearance as there will be no change in pH in the absence of lactose fermentation.
• Example of Lac negative species: Salmonella, Proteus, Yersinia, Pseudomonas
• No colonies:
• Gram-positive bacteria will not form any colonies on MacConkey medium. 
• Slow:
• Weak lactose fermenters will form colonies slower than the rest.  
• Example of slow lac fermenters: Serratia, Citrobacter 
• Mucoid: (sticky, wet colonies)
• Encapsulated bacteria produce capsules using lactose. This gives sticky, wet-appearing colonies.
• Example of mucoid colony-forming species: Klebsiella, enterobacter

Clinical Significance

A pure bacterial culture remains crucial to assess its virulence, its susceptibility to antibiotic therapy, and its genome sequence in order to facilitate the understanding and treatment of caused diseases. MAC's selective and differentiating properties as a culture medium allow isolating colonies of pure bacterial culture from a source sample. Typically in a clinical setting, the collected sample is placed on a panel of many growth media for identification and isolation of bacterial cultures.[3] MAC is one of the widely used growth media as it can work to selectively grow gram-negative bacteria and further differentiate them based on their fermentation profile. Many pathogenic gram-negatives can be differentiated by MAC, especially bacterial gastroenteritis causing species. The diagnostic potential is immense. MAC is essentially a versatile foundation, in which additional substrates, such as sorbitol, can be incorporated for further differentiation. Another example is the addition of antibiotics for testing drug resistance. In intensive care units, testing multi-drug resistance in gram-negative bacteria is an important surveillance measure.[4] Gram-negative enteric bacteria are a common cause of bacterial gastroenteritis, which is characterized by diarrhea, vomiting, and abdominal cramping. Escherichia coli and Campylobacter jejuni are some of the common causes of bacterial gastroenteritis. When bacterial gastroenteritis is suspected, the patient's specimen can be sampled and cultured on a panel of bacterial cultures, which includes MacConkey medium. MAC contributes to the identification of the causal agent by providing lactose-fermentation profiles in gram-negative species.[5]

Nursing, Allied Health, and Interprofessional Team Interventions

A pure bacterial culture remains essential for the study of its virulence, its antibiotic susceptibility, and its genome sequence in order to facilitate the understanding and treatment of caused diseases. Bacterial cultures are still widely used laboratory techniques that many physicians rely on to make their final diagnoses. Many health care professionals are involved in this process, and proper communication and coordination will enhance patient-centered care, improve outcomes, patient safety, and enhance team performance. Bacterial culture from a patient's specimen is a commonly ordered test that many physicians use to aid their medical decision-making process. Nurses are frequently involved in obtaining and transporting samples from the patient to the laboratory. Clinical microbiologists must be trained with a thorough understanding of the principles to run the tests, interpret the results, and report the findings. Sterile sampling techniques and proper handling of the specimen during handling and transport of samples are required to avoid contamination. Any healthcare professionals handling these patient samples must be adequately trained in order to avoid contamination of the samples or accidental exposure and spread of potential pathogens.[6][7]