Differences between Gram-negative and Gram-positive bacteria are fairly straightforward, related to cell membrane structure. Knowing these comparisons give biologists and hobbyists a much richer experience when studying bacteria.
The terms “Gram-negative” and “Gram-positive” come from the stain method developed by Hans Christian Gram in the 1880s. This technique involves applying a series of dyes to bacterial samples to determine the structural differences in their cell walls.
Gram-Positive Bacteria
These bacteria have thick peptidoglycan layers in their cell walls and are able to retain the crystal violet stain used in the Gram staining process. As a result, they look purple under a microscope after staining.
In Gram-positive bacteria the peptidoglycan layer measures between 20-80 nanometers. It provides significant structural support and helps bacteria maintain their shape. Absence of an outer membrane makes them susceptible to external attacks by predators and medications.
Antibiotics such as penicillin target the thick peptidoglycan layer. About 95% of strains of Staphylococcus aureus are sensitive to penicillin.
Examples of Gram-positive bacteria include Streptococcus pneumoniae, a common cause of pneumonia. Staphylococcus aureus is a Gram-positive bacterium responsible for skin infections, and also used as a deep yellow pigment.
Above: Staphylococcus aureus
Lactic acid bacteria (LAB) are a group of Gram-positive, non-spore forming organisms. With high tolerance for low pH, lactic acid bacteria are essential for making cheese, pickles and yogurt.
In the environment, many Gram-positive bacteria are important to decomposition, helping break down organic matter and releasing nutrients back into ecosystems. Bacillus species for example are known to eat through materials like cellulose, a vital process in soil health.
Sporosarcina pasteurii munches on limestone to produce calcite. Brevibacterium linens is instrumental in creation of cheese and body odor. Actinomyces, Clostridium, Mycobacterium, Streptococci, Staphylococci, and Nocardia are all Gram-positive bacteria.
Gram-Negative Bacteria
Gram-negative bacteria have a thin peptidoglycan layer and an outer membrane containing lipopolysaccharides (LPS), better known today as endotoxins. This structure prevents them from retaining the crystal violet dye.
Instead, they take up the counterstain (safranin) and appear pink. Notable Gram-negative bacteria include Escherichia coli, typically found in the intestines, and Salmonella, which causes food poisoning.
Gram-negative bacteria are pivotal in biogeochemical cycles, particularly in nitrogen fixation and pollutant degradation. For instance, Pseudomonas putida is effective in breaking down toxic waste in oil spills, demonstrating its essential role in maintaining ecological balance.
Another Gram-negative bacterium is Cupriavidus metallidurans. It thrives in toxic environments and excretes visible 24k gold. Acetobacter are Gram-negative bacteria without whom we would have no vinegar.
Serratia marcescens causes urinary tract infections and is also used to obtain a bright red pigment. Acidithiobacillus species are extremophiles who thrive in natural toxic zones and create sulfuric acid. Xanthomonas campestris can cause plant blight or make xanthan gum.
Predatory bacterium Vampirovibrio also belongs to the Gram-negative set. This tiny obligate parasite attaches to cells such as algae. It punctures the cell's membrane to suck out the nutritious matter within.
Above: Vampirovibro (arrow) attached to Chlorella sorokiniana
Facts about G+ and G- bacteria
Diversity: Gram-positive bacteria, such as those in the genera Lactobacillus and Bifidobacterium, are essential for digestive health and commonly found in probiotics. Gram-negative bacteria are more diverse, displaying a greater range of structures and functions. Gram-positive bacteria are more numerous but don't show the same variation.
Antibiotic Resistance: The rise of antibiotic-resistant Gram-negative bacteria, exemplified by strains like Enterobacteriaceae, represents one of the most serious health threats globally.
A significant number of antibiotics are derived from natural compounds produced by Gram-positive bacteria, particularly from the genus Streptomyces. Over 60% of antibiotics currently used are derived from Streptomyces species.
Environmental Applications: Some Gram-negative bacteria are used in bioremediation efforts. Their actions help clean up oil spills and break down pollutants.
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