Gram Positive vs. Gram Negative Bacteria

Gram Positive vs. Gram Negative Bacteria Most bacteria are classified into two broad categories: Gram positive and Gram negative. These categories are based on their cell wall composition and reaction to the Gram stain test. The Gram staining method, developed by Hans Christian Gram, identifies bacteria based upon the reaction of their cell walls to certain dyes and chemicals. The differences between Gram positive vs Gram negative bacteria are primarily related to their cell wall composition. Gram positive bacteria have cell walls composed mostly of a substance unique to bacteria known as peptidoglycan, or murein. These bacteria stain purple after Gram staining. Gram negative bacteria have cell walls with only a thin layer of peptidoglycan and an outer membrane with a lipopolysaccharide component not found in Gram positive bacteria. Gram negative bacteria stain red or pink after Gram staining. Gram Positive Bacteria The cell walls of Gram positive bacteria differ structurally from the cell walls of Gram negative bacteria. The primary component of bacterial cell walls is peptidoglycan. Peptidoglycan is a macromolecule composed of sugars and amino acids that are assembled structurally like woven material. The amino sugar component consists of alternating molecules of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM). These molecules are crosslinked together by short peptides which help give peptidoglycan strength and structure. Peptidoglycan provides protection for bacteria and defines their shape. This image shows the cell wall composition of Gram positive bacteria. CNX OpenStax/Wikimedia Commons/CC BY-SA 4.0 The Gram positive cell wall has several layers of peptidoglycan. The thick layers of peptidoglycan help to support the cell membrane and provide a place of attachment for other molecules. The thick layers also enable Gram positive bacteria to retain most of the crystal violet dye during Gram staining causing them to appear purple. Gram positive cell walls also contain chains of teichoic acid that extend from the plasma membrane through the peptidoglycan cell wall. These sugar-containing polymers assist in maintaining cell shape and play a role in proper cell division. Teichoic acid helps some Gram positive bacteria to infect cells and cause disease. Some Gram positive bacteria have an additional component, mycolic acid, in their cell walls. Mycolic acids produce a waxy outer layer that provides additional protection for mycobacteria, such as Mycobacterium tuberculosis. Gram positive bacteria with mycolic acid are also called acid-fast bacteria because they require a special staining method, known as acid-fast staining, for microscope observation. Pathogenic Gram positive bacteria cause disease by the secretion of toxic proteins known as exotoxins. Exotoxins are synthesized within the prokaryotic cell and released into the exterior of the cell. They are specific to certain bacterial stains and can cause serious damage to body organs and tissues. Some Gram negative bacteria also produce exotoxins. Gram Positive Cocci Gram positive cocci refer to Gram positive bacteria that are spherically shaped. Two genera of Gram positive cocci noted for their role as human pathogens are Staphylococcus and Streptococcus. Staphylococcus are spherical in shape and their cells appear in clusters after they divide. Streptococcus cells appear as long chains of cells after division. Examples of Gram positive cocci that colonize the skin include Staphylococcus epidermidis, Staphylococcus aureus, and Streptococcus pyogenes. Staphylococcus aureus is a Gram-positive coccus (round) bacteria that is found on the skin and mucous membranes of humans and many animals. The bacteria are usually harmless, but infections can occur on broken skin or within a blocked sweat or sebaceous gland, resulting in boils, pustules and abscesses. Paul Gunning/Science Photo Library/Getty Images While all three are part of the normal human microbiota, they can cause disease under certain conditions. Staphylococcus epidermidis form thick biofilms and can cause infections associated with implanted medical devices. Some Staphylococcus aureus strains, such as methicillin-resistant Staphylococcus aureus (MRSA), have become resistant to antibiotics and can lead to the development of serious illness. Streptococcus pyogenes can cause strep throat, scarlet fever, blood poisoning, and flesh-eating disease. Gram Negative Bacteria Like Gram positive bacteria, the Gram negative bacterial cell wall is composed of peptidoglycan. However, the peptidoglycan is a single thin layer compared to the thick layers in Gram positive cells. This thin layer does not retain the initial crystal violet dye but picks up the pink color of the counterstain during Gram staining. The cell wall structure of Gram negative bacteria is more complex than that of Gram positive bacteria. Located between the plasma membrane and the thin peptidoglycan layer is a gel-like matrix called periplasmic space. Unlike in Gram positive bacteria, Gram negative bacteria have an outer membrane layer that is external to the peptidoglycan cell wall. Membrane proteins, murein lipoproteins, attach the outer membrane to the cell wall. This image shows the cell wall composition of Gram negative bacteria. CNX OpenStax/Wikimedia Commons/CC BY-SA 4.0 Another unique characteristic of Gram negative bacteria is the presence of lipopolysaccharide (LPS) molecules on the outer membrane. LPS is a large glycolipid complex that protects bacteria from harmful substances in their environment. It is also a bacterial toxin (endotoxin) that can cause inflammation and septic shock in humans if it enters the blood. There are three components of the LPS: Lipid A, a core polysaccharide, and an O antigen. The lipid A component attaches the LPS to the outer membrane. Attached to the lipid A is the core polyssaccharide. It is located between the lipid A component and the O antigen. The O antigen component is attached to the core polyssaccharide and differs between bacterial species. It can be used to identify specific strains of harmful bacteria. Gram Negative Cocci Gram negative cocci refer to Gram negative bacteria that are spherically shaped. Bacteria of the genus Neisseria are examples of Gram negative cocci that cause disease in humans. Neisseria meningitidis is diplococcus, meaning that its spherical cells remain in pairs after cell division. Neisseria meningitidis causes bacterial meningitis and can also cause septicemia and shock. Neisseria meningitidis are spherical, Gram negative bacteria which cause meningitis in humans. The bacteria are typically seen in pairs, each one concave on the side facing its partner. Health Protection Agency/Science Photo Library/Getty Images Another diplococcus bacterium, N. gonorrhoeae, is the pathogen responsible for the sexually transmitted disease gonorrhea. Moraxella catarrhalis is a Gram negative diplococcus that causes ear infections in children, upper respiratory system infections, endocarditis, and meningitis. Gram negative coccobacillus bacteria have bacterial shapes that are in between spherical and rod shaped. Bacteria of the genus Haemophilus and Acinetobacter are coccobacilli that cause serious infections. Haemophilus influenzae can cause meningitis, sinus infections, and pneumonia. Acinetobacter species cause pneumonia and wound infections. Key Points: Gram Positive vs. Gram Negative Bacteria Most bacteria can be broadly classified as Gram positive or Gram negative.Gram positive bacteria have cell walls composed of thick layers of peptidoglycan.Gram positive cells stain purple when subjected to a Gram stain procedure.Gram negative bacteria have cell walls with a thin layer of peptidoglycan. The cell wall also includes an outer membrane with lipopolysaccharide (LPS) molecules attached.Gram negative bacteria stain pink when subjected to a Gram stain procedure.While both Gram positive and Gram negative bacteria produce exotoxins, only Gram negative bacteria produce endotoxins. Sources Silhavy, T. J., et al. The Bacterial Cell Envelope. Cold Spring Harbor Perspectives in Biology, vol. 2, no. 5, 2010, doi:10.1101/cshperspect.a000414.Swoboda, Jonathan G., et al. Wall Teichoic Acid Function, Biosynthesis, and Inhibition. ChemBioChem, vol. 11, no. 1, June 2009, pp. 35–45., doi:10.1002/cbic.200900557.

The Greek Colony of Miletus

The Greek Colony of Miletus Miletus was one of the great Ionian cities in southwestern Asia Minor. Homer refers to the people of Miletus as Carians. They fought against the Achaeans (Greeks) in the Trojan War. Later traditions have Ionian settlers taking the land from the Carians. Miletus itself sent off settlers to the Black Sea area, as well as the Hellespont. In 499 Miletus led the Ionian revolt that was a contributing factor in the Persian Wars. Miletus was destroyed 5 years later. Then in 479, Miletus joined the Delian League, and in 412 Miletus revolted from Athenian control offering a naval base to the Spartans. Alexander the Great conquered Miletus in 334 B.C.; then in 129, Miletus became part of the Roman province of Asia. In the 3rd Century A.D., Goths attacked Miletus, but the city continued, waging an ongoing fight against the silting of its harbor. Early Inhabitants of Miletus The Minoans abandoned their colony in Miletus by 1400 BC. Mycenaean Miletus was a dependency or ally of Ahhiwaya though its population was mostly Carian. Shortly after 1300 BC, the settlement was destroyed by fire- probably at the instigation of the Hittites who knew the city as Millawanda. The Hittites fortified the city against possible naval attacks by the Greeks. Age of the Settlement at Miletus Miletus was regarded as the oldest of the Ionian settlements, though this claim was disputed by Ephesus. Unlike its near neighbors, Ephesus and Smyrna, Miletus was protected from landward assaults by a mountain range and developed early as a sea power. During the 6th century, Miletus contested (unsuccessfully) with Samos for possession of Priene. In addition to producing philosophers and historians, the city was famous for its purple dye, its furniture, and the quality of its wool. The Milesians made their own terms with Cyrus during his conquest of Ionia, though they joined in the rebellion of 499. The city did not fall to the Persians until 494 at which time the Ionian Revolt was considered to be well and truly over. Rule of Miletus Though Miletus was originally ruled by a king the monarchy was overthrown early on. Around 630 BCE a tyranny evolved from its elected (but oligarchic) chief magistracy the prytaneia. The most famous Milesian tyrant was Thrasybulus who bluffed Alyattes out of attacking his city. After the fall of Thrasybulus there came a period of bloody stasis and it was during this period that Anaximander formulated his theory of opposites. When the Persians finally sacked Miletus in 494 they enslaved most of the population and deported them to the Persian Gulf, but there were enough survivors to play a decisive part in the battle of Mycale in 479 (Cimons liberation of Ionia). The city itself, however, was completely razed. The Port of Miletus Miletus, though one of the most famous ports of antiquity is now marooned in an alluvial delta. By the middle of the 5th century, it had recovered from Xerxes attack and was a contributing member of the Delian League. The 5th-century city was designed by the architect Hippodamas, a native of Miletus, and some of the extant remains date from that period. The present form of the theater dates to 100 A.D., but it had existed in an earlier form. It seats 15,000 and faces what used to be the harbor. Source Sally Goetsch of Didaskalia provided notes for this article. Percy Neville Ure, John Manuel Cook, Susan Mary Sherwin-White, and Charlotte Rouechà © Miletus The Oxford Classical Dictionary. Simon Hornblower and Anthony Spawforth. Oxford University Press (2005).