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Introduction to bacteria

Bacterial structure and functions




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USMLE® Step 1 questions

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High Yield Notes

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USMLE® Step 1 style questions USMLE

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A 43-year-old man presents to an urgent care for evaluation of fever, chills, and fatigue which began around three weeks ago. The fever would get progressively higher from day to day, then resolve before redeveloping a few days later. Five days ago, the patient also developed dull headaches. The patient works as a dairy farmer and frequently interacts with livestock. Temperature is 38.4 °C (101.1 °F), blood pressure is 116/78 mmHg, pulse is 102/min and respirations are 18/min. Pulmonary and cardiac examinations are unremarkable. Abdominal examination is notable for hepatosplenomegaly. Which of the following characteristics is true of the pathogen most likely causing this patient’s symptoms?   


Brucella is a genus of Gram-negative coccobacilli, and it groups together several species including Brucella abortus, Brucella melitensis, Brucella canis and Brucella suis.

All cause a systemic disease called brucellosis, but each of them has a different host, and causes a different form of the disease.

Now, Brucella it’s a non-motile bacteria that doesn’t form spores.

It’s a strict aerobe, meaning that it needs oxygen to survive, and also, it’s facultative intracellular which means it can survive both inside and outside the cell.

Finally, it’s urease and catalase positive which means it produces both these enzymes.

Brucella is usually isolated on blood cultures, with biphasic methods like the Ruiz-Castaneda methods.

Biphasic means that the blood culture bottle has both a solid phase, and a liquid phase.

Now, Brucella is a very slow growing bacteria, so colonies usually grow in the solid medium after 6 to 8 weeks of incubation.

The colonies are raised, convex with smooth, shiny corners.

On the bright side, there are now some modern automated blood culture systems called the Bactec systems, which are more effective and can isolate Brucella after only 1 week. Neat!

Now, Brucella can enter the body one of two ways.

First, there may be direct contact with infected animals - and the host is different for each Brucella species.

So, B. abortus is transmitted by cattle, B. melitensis is transmitted by small ruminants such as goats and sheep, B. canis is transmitted by dogs and B. suis is transmitted by swine and rodents.

In this case the bacteria enters through skin lesions, mucous membranes and inhalation.

The second way is ingestion of contaminated animal products such as unpasteurized milk, cheese and undercooked meat.

So, once the bacteria is inside the bloodstream, it’s ingested by phagocytes like macrophages and neutrophils.

Normally, phagocytes destroy invading bacteria by wrapping them up in vesicles called phagosomes, which will merge with lysosomes to form a phagolysosome.

Lysosomes are round vesicles that contain hydrolytic enzymes, which are released inside the phagolysosome to destroy the invading bacteria.

However, Brucella has a few virulence factors that it uses to avoid destruction.


Brucella is a gram-negative coccobacilli that can infect humans and animals. It is the cause of brucellosis, which is a zoonotic disease that can lead to fever, arthritis, and death. There are four main species of Brucella that cause disease in humans: Brucella abortus, Brucella melitensis, Brucella suis, and Brucella canis. Symptoms of brucellosis include fever, headache, myalgia, fatigue, and there may be lymphadenopathy and hepatosplenomegaly.


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