# Gram-Positive vs Gram-Negative Bacteria: USMLE

> What is the difference between gram-positive and gram-negative bacteria?

How the cell envelope decides Gram stain colour, endotoxin and drug coverage, with identification tables for 29 organisms and the classic vignette clues.

Gram-positive bacteria have a thick peptidoglycan wall, roughly 20-80 nm, that traps the crystal violet-iodine complex, so they stain purple. Gram-negative bacteria have a thin peptidoglycan layer beneath an outer membrane rich in lipopolysaccharide; the decolorizer washes the dye out and the counterstain turns them pink. That membrane supplies endotoxin and keeps vancomycin out.

## What is the difference between gram-positive and gram-negative bacteria?

The difference is the cell envelope. A gram-positive bacterium surrounds a single cytoplasmic membrane with a thick peptidoglycan wall, often threaded with teichoic acids. A gram-negative bacterium has two membranes, the inner cytoplasmic membrane and an outer membrane whose outer leaflet is made mainly of lipopolysaccharide (LPS), with a thin peptidoglycan layer in the periplasmic space between them.

Three exam-relevant consequences follow from that design: the colour on a Gram stain, whether the organism carries endotoxin, and which antibiotics can reach their target. More guides on these themes are gathered on the [Infectious Disease](/topics/infectious-disease) hub.

| Feature | Gram-positive | Gram-negative |
| --- | --- | --- |
| Peptidoglycan | Thick, about 20-80 nm | Thin, roughly 2-10 nm |
| Outer membrane | None | Present; outer leaflet mostly LPS |
| Periplasmic space | No outer-membrane-bounded periplasm | Present; holds binding proteins and, in resistant strains, β-lactamases |
| Teichoic and lipoteichoic acids | Present in many species | Absent |
| Endotoxin (lipid A) | None | Lipid A anchors every LPS molecule |
| Porins | None, because there is no outer membrane | Channels that admit hydrophilic molecules up to about 600-700 Da |
| Gram stain colour | Purple | Pink |
| Envelope-specific drugs | Vancomycin and daptomycin act only on gram-positives | Polymyxins bind lipid A in the outer membrane |

## How the Gram stain works, step by step

The Gram stain uses four reagents in a fixed order, and the third, decolorization, is the step that separates the two groups.

1. **Crystal violet**, the primary stain, colours every cell purple.
2. **Iodine**, the mordant, binds the dye into a crystal violet-iodine complex that resists removal.
3. **Alcohol or acetone**, the decolorizer, dehydrates the thick gram-positive wall and closes its pores, trapping the complex. In gram-negative cells the solvent dissolves the lipid outer membrane, and the thin peptidoglycan cannot hold the dye.
4. **Safranin or basic fuchsin**, the counterstain, colours the now-colourless gram-negative cells pink.

Technique and specimen age cause the classic misreads. Prolonged decolorization can strip the dye from gram-positive cells, and old cultures that have lost peptidoglycan can stain gram-negative or gram-variable.

## The cell envelope, layer by layer

Peptidoglycan is the scaffold both groups share: strands of alternating N-acetylglucosamine and N-acetylmuramic acid, cross-linked through short peptides attached to the muramic acid residues. Penicillin-binding proteins carry out the final transpeptidase cross-linking step and are the targets of β-lactams, while vancomycin forms a complex with the terminal D-alanine residues of the peptide precursors.

**Gram-positive extras.** Wall teichoic acids are covalently linked to the peptidoglycan, and lipoteichoic acids are anchored in the cytoplasmic membrane. Surface proteins sit in the same wall, such as the antiphagocytic M protein of *Streptococcus pyogenes*.

**Gram-negative extras.** Three structures have no gram-positive counterpart:

- The **outer membrane**, an asymmetric bilayer with LPS in its outer leaflet, tethered to the peptidoglycan by Braun's lipoprotein.
- **Porins**, outer-membrane channels that limit passive entry to hydrophilic molecules of roughly 600-700 daltons or less.
- The **periplasmic space**, which holds transport binding proteins and is where plasmid-encoded β-lactamases and aminoglycoside-modifying enzymes attack drugs in transit to their targets.

**LPS has three domains:** lipid A, the endotoxic part recognized by the TLR4-MD-2 receptor; a core oligosaccharide; and the O-antigen polysaccharide. *Neisseria* and *Campylobacter* make a truncated form called lipooligosaccharide (LOS).

**Mycobacteria use a third design.** Their wall links very long-chain mycolic acids to arabinogalactan and carries waxes and glycolipids. That chemistry makes them acid-fast: in Ziehl-Neelsen or Kinyoun staining they hold carbol fuchsin through acid-alcohol decolorization and appear red on a blue background.

## Bacteria that stain poorly or not at all

Several high-yield organisms are invisible or misleading on a routine Gram stain, and the exam asks which alternative method reveals each one.

| Organism | Why the Gram stain is unhelpful | How it is shown instead |
| --- | --- | --- |
| *Mycobacterium tuberculosis* | Waxy, mycolic acid-rich wall; effectively gram-neutral | Acid-fast stain (Ziehl-Neelsen or Kinyoun) |
| *Mycoplasma pneumoniae* | No cell wall at all | PCR, the test of choice |
| *Treponema pallidum* | Too slender for conventional light microscopy | Dark-field microscopy; RPR or VDRL plus FTA-ABS serology |
| *Chlamydia* and *Rickettsia* | Very small, obligately intracellular | Giemsa or iodine shows chlamydial inclusions |
| *Legionella pneumophila* | Stains faintly with safranin | Urine antigen (serogroup 1 only) paired with respiratory culture on buffered charcoal yeast extract or molecular testing |

## Gram-positive organisms and their identifying tests

Gram-positive identification starts with shape, then one enzyme test. Among cocci, **catalase** separates staphylococci (positive, grape-like clusters) from streptococci and enterococci (negative, pairs and chains). Staphylococci then split on **coagulase**, which marks *S. aureus*, and the coagulase-negative species split on **novobiocin**. Streptococci split on **hemolysis**: β-hemolysis is complete lysis around the colony, α-hemolysis a partial "greening." Among rods, ask whether the organism forms **spores** (*Bacillus*, aerobic or facultative; *Clostridium*, anaerobic) and whether it **branches** (*Actinomyces*, anaerobic or microaerophilic; *Nocardia*, aerobic).

| Organism | Shape and key tests | Classic presentation |
| --- | --- | --- |
| *Staphylococcus aureus* | Clusters; catalase +, coagulase + | Skin abscesses, pneumonia, endocarditis, septic arthritis, osteomyelitis, toxic shock, food poisoning |
| *Staphylococcus epidermidis* | Clusters; coagulase −, novobiocin sensitive | Infections of prosthetic devices |
| *Staphylococcus saprophyticus* | Clusters; coagulase −, novobiocin resistant | Second most common cause of uncomplicated UTI |
| *Streptococcus pneumoniae* | Lancet-shaped diplococci; α-hemolytic, optochin sensitive, bile soluble | Leading cause of bacterial pneumonia; otitis media, sinusitis, meningitis |
| Viridans streptococci | Chains; α-hemolytic, optochin resistant, bile insoluble | Mouth flora; dental caries (*S. mutans*) and endocarditis |
| *Streptococcus pyogenes* (group A) | Chains; β-hemolytic, bacitracin sensitive | Pharyngitis, impetigo, cellulitis, necrotizing fasciitis; later rheumatic fever or glomerulonephritis |
| *Streptococcus agalactiae* (group B) | Chains; β-hemolytic, CAMP +, hippurate + | Neonatal sepsis and meningitis |
| *Enterococcus* | Pairs and chains; bile esculin +, grows in 6.5% NaCl, PYR + | Hospital UTI, subacute endocarditis, intra-abdominal infection |
| *Bacillus anthracis* | Spore-forming rod; poly-D-glutamic acid capsule | Painless skin ulcer with a black eschar |
| *Bacillus cereus* | Spore-forming rod | Vomiting after rice held at room temperature, then fried; a separate diarrheal form |
| *Clostridium* species and *Clostridioides difficile* | Spore-forming anaerobic rods | Tetanus, botulism, gas gangrene, *C. difficile* colitis |
| *Corynebacterium diphtheriae* | Club-shaped rods, no spores | Pseudomembranous pharyngitis |
| *Listeria monocytogenes* | Motile rods, no spores; grows at 4 °C | Febrile illness with fetal infection in pregnancy; sepsis and meningitis in newborns, older adults and immunosuppression |
| *Actinomyces* | Anaerobic or microaerophilic branching filaments; not acid-fast; sulfur granules | Cervicofacial abscess with draining sinus tracts |
| *Nocardia* | Aerobic branching filaments; weakly acid-fast | Pneumonia, brain abscess, skin infection, often in patients on corticosteroids |

## Gram-negative organisms and their identifying tests

Gram-negative identification starts with shape, then two bench tests. Lactose fermenters such as *E. coli* and *Klebsiella* acidify MacConkey agar and form pink colonies, and the coliforms and *Proteus* are oxidase-negative glucose fermenters. A positive oxidase test points instead toward *Pseudomonas*, *Vibrio*, *Campylobacter*, *Helicobacter* or *Moraxella*.

| Organism | Shape and key tests | Classic presentation |
| --- | --- | --- |
| *Neisseria meningitidis* | Diplococci; polysaccharide capsule; acid from glucose and maltose | Meningitis with petechiae; fulminant Waterhouse-Friderichsen syndrome |
| *Neisseria gonorrhoeae* | Diplococci; no polysaccharide capsule; lipooligosaccharide | Urethritis, cervicitis, PID; diplococci inside neutrophils in urethral pus |
| *Moraxella catarrhalis* | Diplococci; oxidase + | Otitis media, sinusitis |
| *Haemophilus influenzae* | Coccobacilli; needs factors X and V; grows on chocolate agar | Type b: meningitis, epiglottitis, bacteremia; nontypeable: otitis, sinusitis, pneumonia |
| *Escherichia coli* | Rods; lactose fermenter; oxidase − | Leading cause of UTI; neonatal meningitis (K1 capsule) |
| *Klebsiella pneumoniae* | Rods; lactose fermenter; large polysaccharide capsule | Upper-lobe pneumonia with "currant jelly" sputum in alcohol use disorder or diabetes |
| *Proteus mirabilis* | Rods; non-lactose fermenter; urease +; swarming | UTI with alkaline urine and struvite stones |
| *Salmonella* | Rods; flagellated and motile | Gastroenteritis from poultry and eggs; enteric (typhoid) fever with *S.* Typhi |
| *Shigella* | Rods; non-lactose fermenter; non-motile | Dysentery from as few as 10-100 organisms; HUS with *S. dysenteriae* type 1 |
| *Pseudomonas aeruginosa* | Rods; non-fermenter; oxidase +; pyocyanin, fruity odor | Burns, cystic fibrosis, ventilator pneumonia, otitis externa, hot-tub folliculitis |
| *Vibrio cholerae* | Comma-shaped; oxidase +; grows on TCBS agar | Profuse rice-water diarrhea |
| *Campylobacter jejuni* | Curved; oxidase +; grows at 42 °C | Bloody diarrhea after poultry or raw milk; Guillain-Barré trigger |
| *Helicobacter pylori* | Curved; urease +, oxidase +, catalase + | Gastritis, peptic ulcer, gastric cancer, gastric lymphoma |
| *Bacteroides fragilis* | Obligate anaerobic rods; normal colonic flora | Infection after bowel surgery, trauma or inflammation breaches the mucosa |

## Endotoxin versus exotoxins

Endotoxin is a structural part of the gram-negative outer membrane, while exotoxins are proteins that some gram-positive and some gram-negative species release.

| Feature | Endotoxin | Exotoxins |
| --- | --- | --- |
| Chemistry | Lipid A of LPS, a lipid-polysaccharide complex | Proteins |
| Produced by | Gram-negative bacteria | Some gram-positive and some gram-negative species |
| Release | Cell-bound; freed when bacteria lyse | Released from living bacteria |
| Heat | Heat-stable | Most large exotoxins heat-labile; some small ones heat-stable |
| Effect | Fever, blood pressure changes, shock, complement activation, IL-1 release | Among the most potent poisons per unit weight; each causes a specific syndrome |

The high-yield exotoxins are easiest to learn by mechanism:

- **Diphtheria toxin** and ***Pseudomonas* exotoxin A** ADP-ribosylate elongation factor 2 and halt protein synthesis; the diphtheria toxin gene arrives on a temperate bacteriophage.
- **Cholera toxin** ADP-ribosylates the Gs protein that regulates adenylate cyclase, so cAMP rises and the small bowel pours out rice-water stool.
- **Anthrax edema factor** is itself a calmodulin-dependent adenylate cyclase.
- **Shiga toxin** of *S. dysenteriae* type 1 is a ricin-like cytotoxin that inhibits protein synthesis.
- **Tetanus toxin** blocks release of the inhibitory neurotransmitters glycine and GABA, so agonist and antagonist muscles spasm together. The vaccine is formaldehyde-inactivated toxin (toxoid).
- **Botulinum toxin** blocks acetylcholine release at cholinergic synapses, causing flaccid paralysis; honey is the only food implicated in infant botulism, although soil and dust are more common sources of spores.
- ***C. perfringens* alpha toxin** is a lecithinase (phospholipase C) that drives gas gangrene.
- ***C. difficile* toxins A and B** are an enterotoxin and a cytotoxin; clindamycin exposure is the classic precipitant.
- **Superantigen toxins** of *S. aureus* and *S. pyogenes* over-activate inflammatory cells and cytokines in toxic shock syndrome, which is why clindamycin is added to suppress toxin production.

## What the envelope means for antibiotic choice

Antibiotic coverage follows the envelope, because a drug aimed at a gram-negative organism must first cross the outer membrane or attack it directly. Drug-by-drug detail lives in [antibiotic mechanisms and coverage](/blog/antibiotic-mechanisms-coverage-usmle); these are the structural rules.

- **Vancomycin is a gram-positive drug.** Its large, complex molecule cannot penetrate the gram-negative outer membrane. Oral vancomycin is poorly absorbed, which is why it works for intestinal *C. difficile*.
- **Daptomycin also covers only gram-positives**, and pulmonary surfactant inactivates it, so it is not used for pneumonia.
- **Polymyxins** (colistin, polymyxin B) are cationic and bind the negatively charged lipid A, displacing calcium and magnesium; they are last-line agents for carbapenem-resistant Enterobacteriaceae, limited by nephrotoxicity and neurotoxicity.
- **Gram-negative β-lactamases sit in the periplasm**, between the outer membrane and the penicillin-binding proteins. *Bacteroides fragilis* resists penicillin through β-lactamase, and metronidazole is the anaerobic drug of choice.
- **Porin loss raises resistance:** *Pseudomonas aeruginosa* that lacks the OprD porin gains a basal level of carbapenem resistance, especially to imipenem.
- **No wall, no β-lactam target.** *Mycoplasma* is insensitive to penicillin; macrolides, doxycycline or fluoroquinolones treat it.
- **Intrinsic cephalosporin resistance** belongs to *Listeria*, which is treated with ampicillin, and to enterococci.
- ***C. difficile* has its own rule:** for a first episode, the 2021 SHEA/IDSA update suggests fidaxomicin over a standard vancomycin course, with vancomycin an acceptable alternative; either is preferred to metronidazole.

## How USMLE vignettes give the bug away

Most microbiology stems pair the Gram stain with one exposure or one bench test, and that pair names the organism before any culture result. Age and setting matter most in [meningitis vignettes](/blog/meningitis-bacterial-viral-fungal-usmle), where the likely organism changes with the patient's age.

- **Prosthetic joint or valve infection with coagulase-negative, novobiocin-sensitive cocci:** *S. epidermidis*.
- **Newborn sepsis or meningitis with β-hemolytic, CAMP-positive cocci:** group B *Streptococcus*.
- **Pregnant patient or older adult after deli meat, soft cheese or unpasteurized milk:** *Listeria*.
- **Burn wound, cystic fibrosis, ventilator or hot tub:** *Pseudomonas*.
- **Alcohol use disorder with upper-lobe pneumonia and currant-jelly sputum:** *Klebsiella*.
- **Undercooked poultry and bloody diarrhea, later followed by Guillain-Barré syndrome:** *Campylobacter*.
- **Floppy infant who was fed honey:** *Clostridium botulinum*.
- **Diarrhea after clindamycin or another antibiotic course:** *C. difficile*.
- **Pneumonia after exposure to aerosolized contaminated water:** *Legionella*; send a urine antigen together with a respiratory culture.
- **Healthy adult under 40 with pneumonia, cold agglutinins and no response to amoxicillin:** *Mycoplasma*.
- **Jaw abscess with draining sinus tracts and sulfur granules after oral surgery:** *Actinomyces*.
- **Patient on corticosteroids with a lung lesion and a brain abscess:** *Nocardia*.

> **10-second exam strategy:** Read the colour, then the shape, then the single bench test the stem gives you. Purple clusters: catalase, then coagulase. Purple chains: hemolysis, then optochin, bacitracin or CAMP. Pink rods: lactose on MacConkey, then oxidase. Obvious infection with nothing on the Gram stain: think acid-fast, wall-less, too slender or intracellular.

## Common wrong-answer traps

- **Calling mycobacteria gram-negative.** *M. tuberculosis* is effectively gram-neutral; its mycolic acids make it acid-fast, so order an acid-fast stain.
- **Treating *Mycoplasma* with a β-lactam.** With no cell wall there is no target.
- **Expecting vancomycin to cover a gram-negative rod.** The molecule cannot cross the outer membrane.
- **Assuming only gram-negative bacteria cause shock.** Staphylococcal and streptococcal superantigens cause toxic shock syndrome with no endotoxin involved.
- **Forgetting that cephalosporins miss *Listeria* and enterococci.** Both are intrinsically resistant.
- **Assuming a tetanus survivor is immune.** The toxin is so potent that disease does not confer immunity, so the patient still needs the toxoid vaccine.

## Sources

- StatPearls (NCBI Bookshelf): [Gram staining](https://www.ncbi.nlm.nih.gov/books/NBK562156/), [gram-positive bacteria](https://www.ncbi.nlm.nih.gov/books/NBK470553/), [gram-negative bacteria](https://www.ncbi.nlm.nih.gov/books/NBK538213/), [acid-fast bacteria](https://www.ncbi.nlm.nih.gov/books/NBK537121/), [miliary tuberculosis](https://www.ncbi.nlm.nih.gov/books/NBK562300/), [Mycoplasma pneumonia](https://www.ncbi.nlm.nih.gov/books/NBK430780/), [listeriosis](https://www.ncbi.nlm.nih.gov/books/NBK534838/), [group B Streptococcus](https://www.ncbi.nlm.nih.gov/books/NBK553143/), [Enterococcus](https://www.ncbi.nlm.nih.gov/books/NBK567759/), [Nocardia](https://www.ncbi.nlm.nih.gov/books/NBK560872/), [actinomycosis](https://www.ncbi.nlm.nih.gov/books/NBK482151/), [Klebsiella pneumonia](https://www.ncbi.nlm.nih.gov/books/NBK519004/), [Pseudomonas](https://www.ncbi.nlm.nih.gov/books/NBK557831/), [Bacteroides fragilis](https://www.ncbi.nlm.nih.gov/books/NBK553032/), [toxic shock syndrome](https://www.ncbi.nlm.nih.gov/books/NBK459345/), [vancomycin](https://www.ncbi.nlm.nih.gov/books/NBK459263/), [daptomycin](https://www.ncbi.nlm.nih.gov/books/NBK470407/), [polymyxins](https://www.ncbi.nlm.nih.gov/books/NBK557540/), [Rickettsia prowazekii](https://www.ncbi.nlm.nih.gov/books/NBK448173/)
- Medical Microbiology, 4th ed. (NCBI Bookshelf): [structure](https://www.ncbi.nlm.nih.gov/books/NBK8477/), [bacterial pathogenesis](https://www.ncbi.nlm.nih.gov/books/NBK8526/), [antimicrobial chemotherapy](https://www.ncbi.nlm.nih.gov/books/NBK7986/), [Streptococcus](https://www.ncbi.nlm.nih.gov/books/NBK7611/), [Neisseria](https://www.ncbi.nlm.nih.gov/books/NBK7650/), [Haemophilus](https://www.ncbi.nlm.nih.gov/books/NBK8458/), [Escherichia, Klebsiella and Proteus](https://www.ncbi.nlm.nih.gov/books/NBK8035/), [Salmonella](https://www.ncbi.nlm.nih.gov/books/NBK8435/), [Shigella](https://www.ncbi.nlm.nih.gov/books/NBK8038/), [Pseudomonas](https://www.ncbi.nlm.nih.gov/books/NBK8326/), [Vibrio](https://www.ncbi.nlm.nih.gov/books/NBK8407/), [Campylobacter and Helicobacter](https://www.ncbi.nlm.nih.gov/books/NBK8417/), [Clostridia](https://www.ncbi.nlm.nih.gov/books/NBK8219/), [Bacillus](https://www.ncbi.nlm.nih.gov/books/NBK7699/), [Legionella](https://www.ncbi.nlm.nih.gov/books/NBK7619/), [Treponema](https://www.ncbi.nlm.nih.gov/books/NBK7716/), [Chlamydia](https://www.ncbi.nlm.nih.gov/books/NBK8091/), [Mycobacteria and Nocardia](https://www.ncbi.nlm.nih.gov/books/NBK7812/)
- [Essentials of Glycobiology, 4th ed.: Eubacteria (NCBI Bookshelf)](https://www.ncbi.nlm.nih.gov/books/NBK579948/)
- [Structure and function of OprD protein in Pseudomonas aeruginosa (PubMed)](https://pubmed.ncbi.nlm.nih.gov/22226846/)
- [SHEA/IDSA 2021 focused update on Clostridioides difficile infection](https://www.idsociety.org/practice-guideline/clostridioides-difficile-2021-focused-update/)
- CDC: [Pink Book, tetanus](https://www.cdc.gov/pinkbook/hcp/table-of-contents/chapter-21-tetanus.html), [clinical guidance for Legionella infections](https://www.cdc.gov/legionella/hcp/clinical-guidance/index.html)

Gram stain recognition becomes automatic only after these clusters, chains, and pink rods appear in hundreds of stems. [Practice microbiology identification questions on StepGenie](https://dashboard.stepgenie.app/sign-up) and work vignettes connecting stain to drug.

## Frequently asked questions

### Why do gram-positive bacteria stain purple and gram-negative bacteria stain pink?

Both take up crystal violet, which iodine locks into a crystal violet-iodine complex. The alcohol or acetone decolorizer then dehydrates the thick gram-positive peptidoglycan wall and closes its pores, trapping the complex, so those cells stay purple. In gram-negative cells the decolorizer dissolves the lipid outer membrane and the thin peptidoglycan cannot hold the dye, so they lose it and take up the pink safranin or basic fuchsin counterstain.

### What are common examples of gram-positive and gram-negative bacteria?

Gram-positive cocci include Staphylococcus aureus, Streptococcus pneumoniae, Streptococcus pyogenes and Enterococcus; gram-positive rods include Bacillus, Clostridium, Corynebacterium and Listeria, while Actinomyces and Nocardia form branching filaments. Gram-negative cocci include Neisseria and Moraxella. Gram-negative rods include Escherichia coli, Klebsiella, Proteus, Salmonella, Shigella, Pseudomonas and Bacteroides, plus the curved Vibrio, Campylobacter and Helicobacter and the coccobacillus Haemophilus influenzae.

### Why does vancomycin not work against gram-negative bacteria?

Vancomycin blocks cell wall synthesis by binding the D-alanyl-D-alanine end of peptidoglycan precursors. Because of its large size and complex structure, it cannot penetrate the outer membrane of gram-negative organisms, so it never reaches that target. Gram-positive bacteria have no outer membrane, which is why vancomycin is a gram-positive drug. Oral vancomycin is poorly absorbed, which is why it is used for intestinal Clostridioides difficile infection.

### What is the difference between endotoxin and exotoxin?

Endotoxin is the lipid A portion of lipopolysaccharide in the gram-negative outer membrane. It is cell-bound, heat-stable and released when bacteria lyse, and it causes fever, blood pressure changes and shock. Exotoxins are proteins released from living bacteria by some gram-positive and some gram-negative species. Most large exotoxins are heat-labile, they are among the most potent poisons known, and each causes a specific syndrome such as tetanus, botulism, diphtheria or cholera.

### Which bacteria do not show up well on a Gram stain?

Mycobacterium tuberculosis has a mycolic acid-rich wall, is effectively gram-neutral and needs an acid-fast stain such as Ziehl-Neelsen. Mycoplasma has no cell wall and is not visible on Gram stain, so PCR is the test of choice. Treponema pallidum is too slender for conventional light microscopy and is seen with dark-field microscopy. Chlamydia and Rickettsia are too small for a useful Gram stain, and Legionella stains only faintly.

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