The cell wall and the Gram stain

Peptidoglycan, also called murein, is a single giant molecule woven around the whole bacterium. Its backbone is made of two alternating aminosugars, N-acetylglucosamine and N-acetylmuramic acid, joined into long glycan strands. Short peptide chains hang from the muramic acid units, and neighbouring strands are tied together by cross-links between those chains; in Staphylococcus aureus a pentaglycine bridge joins them, and the enzyme that forms the bond is a transpeptidase anchored in the membrane. Penicillin and other beta-lactams imitate the end of the peptide chain, occupy the transpeptidase and prevent new cross-links, so a growing bacterium builds a wall full of holes and bursts. Lysozyme, the enzyme in tears and saliva, attacks the same layer from a different angle by cutting the bond between the two aminosugars. The Gram-positive wall is also threaded with teichoic acids, negatively charged polymers that help bind cations and are not found in Gram-negative organisms. The thickness of this mesh explains the stain. Alcohol dehydrates the peptidoglycan and shrinks it; in a thick wall the pores close and the bulky crystal violet–iodine complex is physically trapped, whereas a thin wall has too little material to close around it. In Gram-negative organisms the alcohol also dissolves the lipopolysaccharide-rich outer membrane and extracts lipid from it, so the dye leaves freely. That is why the decolourisation step, not the dye, decides the Gram reaction, and why the timing and the thickness of the film are the two variables a laboratory controls most carefully. It also explains Gram-variability: an old culture whose wall has begun to autolyse may hold the dye weakly and give an ambiguous result.

When Gram-negative bacteria multiply in the bloodstream, the lipid A portion of their lipopolysaccharide is the signal that turns a local problem into a general one. Lipid A binds to receptors on monocytes and macrophages and makes them release cytokines — tumour necrosis factor, interleukin-1 and interleukin-6 — into the circulation. Those cytokines act on the hypothalamus to raise the body temperature, producing fever and rigors, and they make blood vessels dilate. Widespread vasodilatation lowers the systemic vascular resistance, so blood pressure falls even though the heart is beating faster; capillaries leak fluid into the tissues, and the combination of vasodilatation, leakage, microthrombosis and poor tissue perfusion produces septic shock. Lactate rises as tissues are starved of oxygen, and myocardial depression may follow. Endotoxin is not destroyed by boiling and is not neutralised by ordinary antibiotics; killing the bacteria can even release a further wave of it, which is why septic shock can worsen in the first hours of correct treatment. Gram-positive organisms can also cause severe bloodstream infection, but their typical local behaviour is different. Many of them — Staphylococcus aureus and Streptococcus pyogenes are the classic examples — provoke a walled-off collection of neutrophils and dead tissue, an abscess, and the pus stays localised until it is drained. They produce enzymes such as coagulase that clot plasma around the infection, and they often secrete exotoxins that act at a distance. The practical difference for the nurse is therefore the pattern to watch for: a Gram-negative infection may present with fever, falling blood pressure, rapid breathing and confusion while the original site looks unimpressive, whereas a Gram-positive infection often declares itself as a painful, swollen, discharging focus. Both require the same discipline — culture before antibiotics, prompt treatment, and repeated measurement of vital signs.

A Gram report is only as trustworthy as the smear it came from, and several quite different problems can make it impossible to read. The commonest laboratory variable is decolourisation: too much alcohol, or alcohol left too long, strips the crystal violet from Gram-positive cells as well, so everything looks Gram-negative; too little leaves the violet in Gram-negative cells, so everything looks Gram-positive. Both errors are controlled by running a known positive and a known negative reference slide alongside the patient’s specimen and by keeping the timing constant. Smear thickness matters as much: a thick, dense film holds the dye and decolourises unevenly, so parts of the same field disagree. A very scanty specimen — few organisms in a large volume of fluid — may simply contain nothing to see, especially in early meningitis, where the cerebrospinal fluid can look clear and the stain be negative even though the patient is infected; a negative stain never excludes infection. The organism itself can mislead. Gram-variable species, such as some Bacillus and Clostridium, and senescent cultures whose wall has begun to autolyse, stain weakly and inconsistently. Mycobacterium tuberculosis has a waxy, mycolic-acid-rich wall that resists the dyes and is not seen on an ordinary Gram stain at all; it needs the Ziehl–Neelsen method. Mycoplasma has no wall; spirochaetes such as Treponema pallidum are too thin to be resolved by bright-field light microscopy and need dark-ground or special stains. Previous antibiotics distort the picture by reducing numbers or changing the shape of the cells. The specimen’s own contents interfere too: blood, mucus, mucin, inflammatory debris and haemolysis can obscure organisms or leave precipitate that mimics them. Finally, the interpretation is only as good as the observer, which is why laboratories rely on trained staff, standard operating procedures, proficiency testing and a control slide with every batch. For the nurse, the messages are concrete: collect the specimen before antibiotics, send it without delay, label it fully, and treat a negative Gram stain as one piece of evidence rather than as proof that no infection exists.

Once the colour is known, the second piece of information is morphology, and together they narrow the field far more than beginners expect. Gram-positive cocci in clusters suggest Staphylococcus; in chains, Streptococcus. Gram-positive rods in a wound may be diphtheroids or Clostridium. Gram-negative diplococci inside neutrophils in cerebrospinal fluid are highly suggestive of Neisseria meningitidis, and Gram-negative diplococci in a urethral smear suggest Neisseria gonorrhoeae. Gram-negative rods raise a wide differential that includes Escherichia coli, Klebsiella, Pseudomonas aeruginosa and many others, and the stain cannot separate them. Large Gram-positive rods with a central spore hint at Bacillus anthracis in the right context, but the spore does not identify it. This is why the stain is called presumptive: it supports a probable diagnosis and guides empirical therapy, while the definitive name comes from culture and identification. In the laboratory, identification may use biochemical reactions, antigen detection, serology, matrix-assisted laser desorption ionisation time-of-flight mass spectrometry, or molecular methods such as polymerase chain reaction and ribosomal RNA sequencing. Susceptibility testing then measures which drugs stop the isolate. Two habit points follow for the nurse. First, the Gram stain and the culture are complementary, not alternatives, so the specimen must be collected before the first antibiotic and transported in the right container without delay. Second, a preliminary report is not a final diagnosis: it should prompt the right treatment and the right precautions, and it should also prompt a question — does the patient fit the picture?