Q1 LONG ANSWER 10M ★ High-yield

Classify micro-organisms. Describe the bacterial cell wall and other structures with their applied importance.

Microbiology · 2nd Professional — Para-Clinical

Classification and General Features

Group	Features
Bacteria	Prokaryotes — no nuclear membrane, no mitochondria, a single circular chromosome, 70S ribosomes, and a peptidoglycan cell wall
Fungi	Eukaryotes with a chitin cell wall and an ergosterol membrane — which is why antifungals are more toxic than antibacterials
Viruses	Obligate intracellular parasites with either DNA or RNA (never both), no cell wall and no ribosomes; they use host machinery entirely
Parasites	Eukaryotic protozoa and helminths
Special groups	Mycoplasma (no cell wall, so resistant to beta-lactams); rickettsiae and chlamydiae (obligate intracellular bacteria)

Bacterial Cell Wall

Feature	Gram positive	Gram negative
Peptidoglycan	Thick, 50–90% of the wall, many cross-linked layers	Thin, 5–10%, one or two layers
Outer membrane	Absent	Present — contains lipopolysaccharide (LPS)
Teichoic acid	Present	Absent
Periplasmic space	Minimal	Prominent — contains beta-lactamases
Porins	Absent	Present — hydrophilic channels through which many antibiotics must pass; their loss is a resistance mechanism
Response to lysozyme	Susceptible	More resistant
Toxin	Chiefly exotoxins (secreted proteins)	Endotoxin (LPS), released on lysis
Antibiotic susceptibility	More susceptible to penicillin and lysozyme	The outer membrane excludes many agents — vancomycin is too large to cross it, which is why gram-negatives are intrinsically resistant

CLINICAL PEARL

The Gram stain works because of what the wall does with the decolouriser. Crystal violet–iodine complex is trapped in the thick, highly cross-linked peptidoglycan of gram-positive organisms, which alcohol dehydrates and closes further. In gram-negatives the alcohol dissolves the lipid-rich outer membrane and the thin peptidoglycan cannot retain the dye, so the complex washes out and the counterstain shows. Almost every difference between the two groups — toxin type, antibiotic susceptibility, lysozyme sensitivity — follows from that same wall structure.

Lipopolysaccharide and Endotoxin

LPS has three parts — lipid A, a core polysaccharide, and the O-specific side chain → lipid A is the toxic moiety; the O side chain is the O antigen used in serotyping (as in Salmonella and E. Coli O157) → On bacterial lysis, lipid A is released and binds TLR4 on macrophages → Massive release of TNF-alpha, IL-1 and IL-6 → Fever, hypotension, capillary leak, disseminated intravascular coagulation → septic shock and multi-organ failure

Feature	Exotoxin	Endotoxin
Nature	Protein, secreted by living cells	Lipopolysaccharide, part of the wall, released on lysis
Source	Mainly gram-positive (also some gram-negative)	Gram-negative only
Heat stability	Heat-labile (destroyed at 60°C)	Heat-stable (withstands autoclaving) — which is why sterile fluids can still be pyrogenic
Potency and specificity	Highly potent and highly specific in action	Less potent; effects are the same whatever the organism
Antigenicity	Strongly antigenic; converted to toxoids for vaccines (tetanus, diphtheria)	Poorly antigenic; NO toxoid can be made
Examples	Tetanospasmin, diphtheria toxin, cholera toxin, botulinum toxin	LPS of Salmonella, E. Coli, Neisseria

Other Structures and Their Significance

Capsule — usually polysaccharide (but polypeptide (D-glutamate) in Bacillus anthracis). It is antiphagocytic and therefore a major virulence factor; demonstrated by the quellung reaction and by negative staining. Capsular polysaccharide is the basis of the pneumococcal, meningococcal and Hib conjugate vaccines
Flagella — organs of motility, made of flagellin; the H antigen. Arrangements: monotrichous, lophotrichous, amphitrichous and peritrichous. Motility is demonstrated by hanging drop or in semisolid medium
Fimbriae (common pili) — short, numerous, for adhesion to host cells; a virulence factor in uropathogenic E. Coli and gonococci. sex pilus — long, one or two per cell, mediates conjugation and the transfer of resistance plasmids
Spores — dormant, highly resistant forms produced by Bacillus and Clostridium; contain dipicolinic acid with calcium and very little water, which is the basis of their heat resistance. They are the reference standard for sterilisation
Plasmids — extrachromosomal circular DNA carrying resistance and virulence genes, transferable between organisms
Cell membrane — site of respiration and transport; sterols are absent except in mycoplasma

Bacterial Morphology and Arrangement

Shape	Arrangement and examples
Cocci (spherical)	In clusters — Staphylococcus; in chains — Streptococcus; in pairs (diplococci) — pneumococcus (lanceolate) and Neisseria (kidney-shaped); in tetrads — Micrococcus; in packets of eight — Sarcina
Bacilli (rod-shaped)	Single, in chains (streptobacilli), in "chinese letter" or palisade arrangement — Corynebacterium; "bamboo stick" appearance — Bacillus anthracis
Coccobacilli	Short ovoid rods — Haemophilus, Brucella, Bordetella
Vibrio	Comma-shaped, with a single polar flagellum; "fish in stream" appearance in a hanging drop — Vibrio cholerae
Spiral	Spirilla (rigid) and spirochaetes (flexible, with axial filaments) — Treponema, Leptospira, Borrelia
Filamentous / branching	Actinomyces, Nocardia — resemble fungi but are bacteria
Pleomorphic	Lacking a fixed shape — mycoplasma, which has no cell wall

Arrangement reflects the plane of division and whether daughter cells separate — division in one plane with cells remaining attached gives chains; division in several planes gives clusters. It is a genuinely useful identifying feature on a Gram film

Applied Aspects

The peptidoglycan wall is absent in man, which is why penicillins and cephalosporins have such a wide therapeutic index — their only serious hazard is allergy
Mycoplasma has no cell wall, so beta-lactams are useless against it; this is why atypical pneumonia requires a macrolide or tetracycline
Endotoxin survives autoclaving, so intravenous fluids and devices must be tested for pyrogens (the LAL test) as well as for sterility — a sterile solution can still cause rigors
Gram-negative sepsis remains a leading cause of death, and the cytokine cascade explains why killing the organism with an antibiotic can transiently worsen the patient through massive endotoxin release
Capsular vaccines fail in infants unless conjugated to a protein carrier, because polysaccharide alone provokes a T-independent response without memory
The Gram stain remains one of the most valuable and cheapest investigations in medicine — available within minutes, it guides initial antibiotic choice long before culture results
L-forms and spheroplasts are cell-wall-deficient variants produced by penicillin or lysozyme; they can revert and may explain some relapses after apparently adequate treatment
Fimbriae explain why urinary infection is so common in women — uropathogenic E. Coli adhere by P fimbriae to uroepithelial receptors, and the short urethra shortens the journey
Motility is a useful bedside laboratory clue — the darting "fish in a stream" motility of Vibrio cholerae in a hanging drop, abolished by specific antiserum, has been used for rapid diagnosis in outbreaks
Antigenic structure underlies serotyping — the O (somatic), H (flagellar) and K or Vi (capsular) antigens are the basis of the Kauffmann-White scheme for Salmonella and of E. Coli serotyping such as O157:H7
Rickettsiae and chlamydiae are bacteria, not viruses, despite being obligate intracellular organisms — they have both DNA and RNA and a cell wall, and they respond to antibiotics, which viruses do not

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