Lactose Fermenter (LF) and Non-Lactose Fermenter (NLF) Gram-Negative Bacteria: Introduction, List of LF and NLF Bacteria, and Keynotes

Introduction of LF and NLF Gram-Negative Bacteria on MacConkey Medium

MacConkey agar/ medium is recommended for use as a selective, differential, and indicator growth medium for the isolation of gram-negative bacilli including coliform organisms and enteric pathogens, on the basis of lactose fermentation. Peptones (meat and casein) and the pancreatic digest of gelatin provide the essential nutrients, vitamins, and nitrogenous factors required for the growth of microbes. Lactose monohydrate is the fermentable source of carbohydrates. The selective action of this medium is contributed by crystal violet and bile salts, which are inhibitory to most species of gram-positive bacteria. Sodium chloride maintains the osmotic balance in the medium whereas neutral red is a pH indicator that turns red at a pH below 6.8 (lactose fermenter) and is colorless at any pH greater than 6.8 (non-lactose fermenter). Agar is the solidifying agent.

List of LF Bacteria and NLF Bacteria

The list of lactose fermenter Gram-negative bacteria on the basis of MacConkey medium are as follows-

S. No.	Organisms	Colony Morphology
1	Escherichia coli	Lactose fermenter
2	Klebsiella spp.	Lactose fermenter
3	Citrobacter spp.	Late lactose fermenter
4	Enterobacter spp.	Lactose fermenter
5	Serratia spp.	Late lactose fermenter
6	Erwinia spp.	Lactose fermenter

The list of non-lactose fermenter Gram-negative bacteria on the basis of MacConkey medium are as follow-

S. No.	Organisms	Colony Morphology
1	Salmonella spp.	Non-Lactose Fermenter
2	Shigella spp.	Non-Lactose Fermenter except S.sonnei(Late lactose fermenter)
3	Proteus spp.	Non-Lactose Fermenter
4	Providencia spp.	Non-Lactose Fermenter
5	Morgenella	Non-Lactose Fermenter
6	Hafnia alvei	Non-Lactose Fermenter
7	E. tarda	Non-Lactose Fermenter
8	Pseudomonas spp.	Non-Lactose Fermenter
9	Acinetobacter spp.	Non-Lactose Fermenter
10	Burkholderia cepacia complex	Non-Lactose Fermenter

Keynotes on Lactose Fermenter (LF) and Non-Lactose Fermenter (NLF) Gram-Negative Bacteria

1. Pasteurella species (including Pasteurella multocida) will not grow on MacConkey Agar.
2. MacConkey agar without bile salt- It uses both gram-negative and gram-positive bacteria
3. MacConkey agar with bile salt- Selective for gram-negative bacteria but Enterococcus species may grow.
4. MacConkey agar with bole sat and crystal violet: Strict selective medium for gram-negative bacteria that also inhibits Entercoccus species due to having crystal violet in its composition.
5. MacConkey Agar with Sorbitol (Sorbitol-MacConkey agar) is to be used as a selective and differential medium for the detection of enterohemorrhagic Escherichia coli O157:H7.
6. Fermenter,oxidizer and non-utilizer bacteria can only be confirmed using oxidative fermentative (OF) test.

LF and NLF Gram-Negative Bacteria

Escherichia coli colony morphology on MacConkey agar

LF and NLF Colonies of Klebsiella and Pseudomonas on MacConkey medium

Mucoid lactose fermenter (MLF) colony of Klebsiella pneumoniae on MacConkey agar

Dangerous mucoid lactose fermenter (MLF) colony of Klebsiella pneumoniae on MacConkey agar

Proteus mirabilis colony characteristics on Macconkey medium showing non-lactose fermenter (NLF) colonies

Salmonella growth on MacConkey agar

Shigella colony morphology on MacConkey agar

Proteus vulgaris growth on MacConkey agar

Pseudomonas aeruginosa colony morphology on MacConkey agar

Mucoid strain of Pseudomonas aeruginosa colony morphology on MacConkey agar

Acinetobacter colony morphology on MacConkey agar

Serratia marcescens growth on Muller-Hinton agar with pigment expression Demonstration

Aeromonas hydrophila colony morphology on MacConkey agar

Alcaligenes faecalis growth on MacConkey agar

Further Reading

• https://www.sciencedirect.com/topics/medicine-and-dentistry/providencia
• http://www.oxoid.com/uk/blue/prod_detail/prod_detail.asp?pr=CM0007
• https://universe84a.com/collection/acinetobacter-2/
• https://www.sciencedirect.com/topics/medicine-and-dentistry/edwardsiella
• https://universe84a.com/macconkey-agar-introduction/
• https://universe84a.com/oxidative-fermentative-test/
• Cowan & Steel’s Manual for identification of Medical Bacteria. Editors: G.I. Barron & R.K. Felthani, 3rd ed 1993, Publisher Cambridge University Press.
• Bailey & Scott’s Diagnostic Microbiology. Editors: Bettey A. Forbes, Daniel F. Sahm & Alice S. Weissfeld, 12th ed 2007, Publisher Elsevier.
• Clinical Microbiology Procedure Handbook, Chief in editor H.D. Isenberg, Albert Einstein College of Medicine, New York, Publisher ASM (American Society for Microbiology), Washington DC.
• https://catalog.hardydiagnostics.com/cp_prod/content/hugo/ofbasalmedium.htm
• Colour Atlas and Textbook of Diagnostic Microbiology. Editors: Koneman E.W., Allen D.D., Dowell V.R. Jr, and Sommers H.M.
• Jawetz, Melnick and Adelberg’s Medical Microbiology. Editors: Geo. F. Brook, Janet S. Butel & Stephen A. Morse, 21st ed 1998, Publisher Appleton & Lance, Co Stamford Connecticut.
• Mackie and Mc Cartney Practical Medical Microbiology. Editors: J.G. Colle, A.G. Fraser, B.P. Marmion, A. Simmous, 4th ed, Publisher Churchill Living Stone, New York, Melborne, Sans Franscisco 1996.
• Textbook of Diagnostic Microbiology. Editors: Connie R. Mahon, Donald G. Lehman & George Manuselis, 3rd edition2007, Publisher Elsevier.