STEPTRACKERMICROBIOLOGY | BACTERIAL BASICS | HIGH-YIELD REVIEW

Bacterial Structure & Cell Wall

Each structure has a job, a chemical makeup and a clinical consequence — from the thick peptidoglycan of gram-positives to the LPS of gram-negatives.

FOCUS ON

Gram-positive: thick peptidoglycan + lipoteichoic acid (stains purple). Gram-negative: thin peptidoglycan, periplasm, outer membrane with LPS (endotoxin) (stains pink). Capsule (polysaccharide; B anthracis is polypeptide) blocks phagocytosis → vaccine target. Spores (Bacillus, Clostridium) have dipicolinic acid, survive boiling — need autoclaving. Mycobacteria have mycolic acids; Mycoplasma has no wall (sterols).

STRUCTURE

COMPOSITION

FUNCTION

CLINICAL CLUE

Peptidoglycan

NAG–NAM sugar backbone cross-linked by peptide bridges (transpeptidases = PBPs)

Rigid support, resists osmotic lysis

β-lactams block PBP cross-linking · vancomycin binds D-Ala-D-Ala

Teichoic / lipoteichoic acid

gram-positive only

Polymers anchored in wall/membrane

Induce TNF-α and IL-1

Gram-positive septic shock trigger

Outer membrane

gram-negative only

Phospholipid inner leaflet, LPS outer leaflet (lipid A + core + O antigen), porins

Barrier to vancomycin, many drugs; endotoxin

Lipid A → TLR4 → septic shock, DIC

Periplasm

gram-negative

Space between inner and outer membranes

Holds β-lactamases, hydrolytic enzymes

Gram-negative resistance reservoir

Capsule

Polysaccharide (except B anthracis: D-glutamate polypeptide)

Antiphagocytic; quellung reaction swells it

Conjugated polysaccharide vaccines (pneumococcus, Hib, meningococcus) · asplenia → encapsulated infections

Glycocalyx / slime

Loose polysaccharide film

Adherence, biofilms on catheters and prostheses

S epidermidis, Pseudomonas in CF, viridans strep on valves

Spores

Keratin-like coat, dipicolinic acid, peptidoglycan, little water

Survive heat, drying, chemicals; no metabolic activity

Only gram-positive rods: Bacillus, Clostridium · autoclave (121 °C) to kill

Pili / fimbriae

Glycoprotein surface projections

Adherence; sex pilus for conjugation

Gonococcal antigenic variation · UPEC P fimbriae

Flagellum

Protein (flagellin)

Motility; H antigen

Proteus swarming · Listeria tumbling (actin rockets intracellular)

Mycolic acids

Very long-chain fatty acids in mycobacterial wall

Waxy, acid-fast, resists drying

Isoniazid blocks mycolic acid synthesis

No cell wall

Mycoplasma (membrane sterols); Ureaplasma

Not seen on Gram stain; β-lactam resistant

Treat with macrolides, tetracyclines

Plasmids

Extrachromosomal circular DNA

Carry resistance and toxin genes

Spread by conjugation (1.05)

Picture it — the two cell envelopes

Gram-positiveA thick peptidoglycan mesh threaded with teichoic acids sits directly on the cell membrane and traps the crystal violet–iodine complex.

Gram-negativeA thin peptidoglycan layer lies in the periplasm between two membranes; the outer one carries LPS and porins, and alcohol washes the stain out.

Schematic diagrams, not to scale.

Mechanism — why the Gram stain works

Crystal violet + iodineAll cells purple

→

Alcohol washThick peptidoglycan traps dye; thin wall + outer membrane lose it

→

SafraninGram-negatives turn pink

Poor Gram staining: no wall (Mycoplasma), too thin (Treponema, Leptospira), intracellular (Chlamydia, Rickettsia, Legionella), or waxy (Mycobacteria).

Gram-positive vs gram-negative

FEATURE

GRAM-POSITIVE

GRAM-NEGATIVE

Peptidoglycan

Thick

Thin

Outer membrane / LPS

Yes (endotoxin)

Teichoic acid

Yes

Periplasm

Yes

Vancomycin works

Yes

No (can't penetrate)

Spores possible

Bacillus, Clostridium

Coxiella (spore-like)

How it's tested

COMMON CONFUSIONEndotoxin (LPS, gram-negative, part of the wall) vs exotoxins (secreted proteins, both gram types).

THE TRAPSpores survive alcohol hand gel — C difficile needs soap-and-water washing and sporicidal cleaning.

YOUR APPROACHIdentify the structure → what it's made of → what it does → which drug, vaccine or test exploits it.

If you see… think…

Gram-negative wall component causing septic shock → lipid A of LPS

Gram-positive wall component that triggers cytokines → lipoteichoic acid

Capsule made of protein (D-glutamate) → Bacillus anthracis

Positive quellung reaction → capsular swelling with antiserum

Survives boiling, killed by autoclave → spore former

Spore core component → dipicolinic acid

Biofilm on a prosthetic joint → S epidermidis

Bacterium with sterols and no wall → Mycoplasma

Waxy wall, acid-fast → mycolic acids

Where β-lactamases sit in gram-negatives → periplasm