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Cell wall
93%
366/393
Mitochondria
1%
5/393
Golgi
0%
1/393
Nucleus
3%
10/393
Endoplasmic reticulum
2/393
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The patient's clinical picture is consistent with S. aureus meningitis. S. aureus looks purple after a gram stain ("gram-positive"), because the cell wall retains the violet dye. The vast majority of bacteria can be divided into gram-positive bacteria and gram-negative bacteria. These terms refer to the color of the bacteria after performing a gram stain, which involves first processing a sample slide with crystal violet dye, and then with a counterstain (often fuchsin). This stain differentiates between bacteria that have a single, membrane bilayer encased with a thick layer of peptidoglycan ("gram-positive"), from bacteria that have two membrane bilayers with a thin layer of peptidoglycan in between ("gram-negative"). This difference is perhaps most clinically relevant when considering antibiotic coverage of different types of organisms. Some antibiotics are not able to penetrate the membrane bilayer of gram-negative bacteria, while other antibiotics are not able to penetrate the thick peptidoglycan layer of gram-positive bacteria. Seehusen et al. discuss the analysis of cerebrospinal fluid. They note that the sensitivity of Gram-staining is dependent on the causal organism, with nearly 90% sensitivity for S. aureus and less than 50% sensitivity for Listeria. Having a higher number of colony forming units increases the chance of finding bacteria on a gram stain. In addition, nearly 10% of Gram stains are misread, and therefore technician reliability should be considered when accounting for a Gram stain. Nigrovic et al. perform a retrospective cohort study to evaluate the validity of the Bacterial Meningitis Score for determining whether children with a cerebrospinal fluid pleocytosis actually have bacterial meningitis. The prediction tool defines those at very low risk for bacterial meningitis as those who have none of the following: "positive CSF Gram stain, CSF absolute neutrophil count (ANC) of at least 1,000 cells/microL, CSF protein of at least 80 mg/dL, peripheral blood ANC of at least 10,000 cells/microL, and a history of seizure before or at the time of presentation." They found that children with a pleocytosis who lack all of the above characteristics have a 0.1% chance of having bacterial meningitis. Figure A shows a Gram stain of S. aureus. Illustration A shows a Gram stain of S. aureus (purple, gram-positive) and E. coli (pink, gram-negative). Illustration B shows the structure of the bacterial cell wall in gram-positive bacteria (A) and gram-negative bacteria (B). Incorrect Answers: Answers 2-5: These bacterial organelles do not affect Gram staining.
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