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Gram-negative bacteria

Gram-negative bacteria is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Gram-negative bacteria rather than just read about it. In short: Gram-negative bacteria are bacteria that, unlike gram-positive bacteria, do not retain the crystal violet stain used in the Gram staining method of bacterial differentiation. Their defining characteristic is that their cell envelope consists of a thin peptidoglycan cell wall sandwiched between an inner (cytoplasmic) membrane and an outer membrane.

Gram-negative bacteria — main illustration
Gram-negative bacteria — illustration

Key takeaways

  • Gram-negative bacteria belongs to science; place it in that map before memorising details.
  • Learn the definition first, then one example that makes the definition concrete.
  • Connect Gram-negative bacteria to a quantity you can measure, compute or draw — that is where exam questions come from.
  • Reproduce the core statement of Gram-negative bacteria from memory before moving on to harder problems.

Reference excerpt

Gram-negative bacteria are bacteria that, unlike gram-positive bacteria, do not retain the crystal violet stain used in the Gram staining method of bacterial differentiation. Their defining characteristic is that their cell envelope consists of a thin peptidoglycan cell wall sandwiched between an inner (cytoplasmic) membrane and an outer membrane. These bacteria are found in all environments that support life on Earth. Within this category, notable species include the model organism Escherichia coli, along with various pathogenic bacteria, such as Pseudomonas aeruginosa, Chlamydia trachomatis, and Yersinia pestis. They pose significant challenges in the medical field due to their outer membrane, which acts as a protective barrier against numerous antibiotics (including penicillin), detergents that would normally damage the inner cell membrane, and the antimicrobial enzyme lysozyme produced by animals as part of their innate immune system. Furthermore, the outer leaflet of this membrane contains a complex lipopolysaccharide (LPS) whose lipid A component can trigger a toxic reaction when the bacteria are lysed by immune cells. This reaction may lead to septic shock, resulting in low blood pressure, respiratory failure, reduced oxygen delivery, and lactic acidosis. Several classes of antibiotics have been developed to target gram-negative bacteria, including aminopenicillins, ureidopenicillins, cephalosporins, beta-lactam-betalactamase inhibitor combinations (such as piperacillin-tazobactam), folate antagonists, quinolones, and carbapenems. Many of these antibiotics also cover gram-positive bacteria. The antibiotics that specifically target gram-negative bacteria include aminoglycosides, monobactams (such as aztreonam), and ciprofloxacin.

Characteristics

Conventional gram-negative (LPS-diderm) bacteria display the following characteristics:

An inner cell membrane is present (cytoplasmic) A thin peptidoglycan layer is present (this is much thicker in gram-positive bacteria) Has outer membrane containing lipopolysaccharides (LPS, which consists of lipid A, core polysaccharide, and O antigen) in its outer leaflet and phospholipids in the inner leaflet Porins exist in the outer membrane, which act like pores for particular molecules Between the outer membrane and the cytoplasmic membrane there is a space filled with a concentrated gel-like substance called periplasm The S-layer is directly attached to the outer membrane rather than to the peptidoglycan If present, flagella have four supporting rings instead of two Teichoic acids or lipoteichoic acids are absent Lipoproteins are attached to the polysaccharide backbone Some contain Braun's lipoprotein, which serves as a link between the outer membrane and the peptidoglycan chain by a covalent bond Most, with few exceptions, do not form spores However, the LPS-diderm group (corresponding to kingdom Pseudomonadati, formerly "Hydrobacteria") is not the only type of bacteria that stain negative. Mycobacterium (or rather most of Mycobacteriales), which does not belong in the group, have independently evolved an outer cell membrane, with a cell wall made of mycolic acid. This gives it very different structure and features. In many gram-negative bacteria, the IgaA membrane protein negatively regulates the Rcs phosphorelay system, a key envelope stress response pathway that helps maintain cell envelope integrity.

Classification Along with cell shape, Gram staining is a rapid diagnostic tool and once was used to group species at the subdivision of Bacteria. Historically, the kingdom Monera was divided into four divisions based on Gram staining: Firmicutes (+), Gracillicutes (−), Mollicutes (0) and Mendocutes (var.). Since 1987, the monophyly of the gram-negative bacteria has been disproven with molecular studies. Current knowledge divides the gram-negative bacteria into two large groups and some straddlers. The more "conventional" with an LPS outer membrane do share a common ancestor and are grouped in kingdom Pseudomonadati. The less conventional ones are the order Mycobacteriales, that have a mycolic acid cell wall and an outer membrane. The kingdom and the order are each monophyletic (or rather, not holyphyletic), but the "LPS-diderm" and "mycolic-diderm" groups are not, because some bacteria in the kingdom and the order do not stain gram negative. They will be discussed in the next section.

Taxonomy

… excerpt ends here. Continue reading the full article.

Illustrations

Gram-negative bacteria: Microscopic image of gram-negative Pseudomonas aeruginosa bacteria (pink-red rods)
Microscopic image of gram-negative Pseudomonas aeruginosa bacteria (pink-red rods)
Gram-negative bacteria: Gram-negative (LPS-diderm) cell wall structure
Gram-negative (LPS-diderm) cell wall structure
Gram-negative bacteria: Gram-positive and gram-negative bacteria are differentiated chiefly by their cell wall structure
Gram-positive and gram-negative bacteria are differentiated chiefly by their cell wall structure
Gram-negative bacteria: Example of a workup algorithm of possible bacterial infection in cases with no specifically requested targets (non-bacteria, mycobacteria etc.), with most common situations and agents seen in a New England setting. Clinically significant gram-negative bacteria are usually rods, as shown near bottom right. Although some gram-negative bacteria can be recognized by "bench tests", diagnosis in the modern microbiology lab usually involves MALDI-TOF and/or multitarget assay.
Example of a workup algorithm of possible bacterial infection in cases with no specifically requested targets (non-bacteria, mycobacteria etc.), with most common situations and agents seen in a New England setting. Clinically significant gram-negative bacteria are usually rods, as shown near bottom right. Although some gram-negative bacteria can be recognized by "bench tests", diagnosis in the modern microbiology lab usually involves MALDI-TOF and/or multitarget assay.

Worked examples

Example 1 — a first encounter with Gram-negative bacteria

Start with the simplest possible case. Write down what Gram-negative bacteria claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Gram-negative bacteria before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Gram-negative bacteria ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Gram-negative bacteria

In research
Gram-negative bacteria appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry
Engineering practice reuses Gram-negative bacteria in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom
Gram-negative bacteria is common in secondary-school and first-year university syllabi. It links to neighbouring topics Bacteriology, Gram-negative bacteria, Staining, so understanding it makes those chapters shorter.
In everyday life
Look for Gram-negative bacteria outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.
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How to study Gram-negative bacteria in 20 minutes

  1. Read the reference excerpt below once, without taking notes.
  2. Close the page and write down what Gram-negative bacteria means in your own words.
  3. Compare your version with the excerpt and mark what you missed.
  4. Work through the three examples above with pen and paper.
  5. Explain Gram-negative bacteria out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Gram-negative bacteria in simple terms?

Gram-negative bacteria are bacteria that, unlike gram-positive bacteria, do not retain the crystal violet stain used in the Gram staining method of bacterial differentiation. Their defining characteristic is that their cell envelope consists of a thin peptidoglycan cell wall sandwiched between an i…

Why does Gram-negative bacteria matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Gram-negative bacteria?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Gram-negative bacteria.

Tags

  • Bacteriology
  • Gram-negative bacteria
  • Staining

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