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FinalMB Part I - Medicine > Infectious Diseases > Flashcards

Flashcards in Infectious Diseases Deck (176)
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Aerobic bacteria require oxygen whereas anaerobic bacteria do not. GRAM POSITIVE bacteria have a thick peptidoglycan cell wall that stains with crystal violet stain. GRAM NEGATIVE bacteria don't have this thick peptidoglycan cell wall and don't stain with crystal violet stain will stain with other stains. ATYPICAL BACTERIA cannot be stained or cultured in the normal way.

Also rod shaped bacteria are called bacilli and circular shaped bacteria are called cocci.



Nucleic acid is an essential component of bacterial DNA

Ribosomes are where bacteria proteins are synthesised within the bacterial cell.

Folic acid is essential for the synthesis and regulation of DNA within the bacteria. Folic acid cannot be directly imported into the cell and requires a chain of intermediates to get in. This chain starts with para-aminobenzoic acid (PABA), which is directly absorbed across the cell membrane and into the cell. PABA is converted to dihydrofolic acid (DHFA), which is converted inside the cell to tetrahydrofolic acid (THFA), then folic acid.



A gram stain is used as a quick way to check a sample under a microscope to look for bacteria. It involves two main steps:

Add a crystal violet stain, which binds to molecules in the thick peptidoglycan cell wall in gram positive bacteria turning them violet.

Then add a counterstain (such as safranin) which binds to the cell membrane in bacteria that don't have a thick, peptidoglycan cell wall (gram negative bacteria) turning them red/pink


Gram positive cocci ?



Gram positive rods ?

Use the mnemonic "corney Mike's list of basic cars":

Corney - Corneybacteria
Mike's - Mycobacteria
List of - Listeria
Basic - Bacillus
Cars - Nocardia


Gram positive anaerobes ?


C - Clostridium
L - Lactobacillus
A - Actinomyces
P - Propionibacterium


Gram negative bacteria ?

Neisseria meningitidis
Neisseria gonorrhoea
Hameophilia influenza
E. coli
Pseudomonas aeruginosa
Moraxella catarrhalis


Definition of an atypical bacteria ?

Bacteria that cannot be cultured in the normal way or detected using a gram stain.


Atypical are most often implicated in what ?



Atypical bacteria that cause atypical pneumonia ?

"Legions of psittaci MCQs"

Legions - Legionella pneumophila
Psittaci - Chlamydia psittaci
M - Mycoplasma pneumoniae
C - Chlamydophilia pneumoniae
Qs - Q fever (coxiella burnetii)


Which type of antibiotics is MRSA resistant to + give examples?

Beta-lactam antibiotics such as penicillins, cephalosporins and carbapenems.


People are often colonised with MRSA bacteria and have them living harmlessly on their skin and respiratory tract. If these bacteria become part of an infection they can be difficult to treat. Pts being admitted for surgery or inpatient treatment are screened for MRSA infection by taking nose and groin swabs, so that extra measures can be taken to try and eradicate them and stop their spread. Eradication involves a combination of what ?

Chlorhexidine body washes and antibacterial nasal creams


Antibiotic treatment options for MRSA are ?



What are extended spectrum beta lactamase bacteria (ESBLs) ?

Bacteria that have developed resistance to beta-lactam antibiotics. They produce beta lactamase enzymes that destroy the beta-lactam ring on the antibiotic. They can be resistant to a very broad range of antibiotics.


ESBLs tend to be + what do they usually cause

E. coli or klebsiella and typically cause UTIs but can also cause other infections such as pneumonia


What are ESBLs sensitive to ?

Carbapenems such as meropenem or imipenem.


What do bacteriostatic antibiotics do ?

They either stop the reproduction or growth of bacteria


What do bactericidal antibiotics do ?

They directly kill bacteria



In your OSCEs questions about treating infections can always be answered with "treat with antibiotics according to the local antibiotic policy"


Antibiotics that inhibit cell wall synthesis ?

Antibiotics with a beta-lactam ring:
-Carbapenems such as meropenem

Antibiotics without a beta-lactam ring:


Antibiotics that inhibit folic acid metabolism ?

-Sulfamethoxazole blocks the conversion of DHFA to THFA
-Trimethoprim blocks the conversion of THFA to folic acid

-Co-trimoxazole is a combination of the two.


Metronidazole ?

The reduction of metronidazole into its active form only occurs in anaerobic cells. When partially reduced, metronidazole inhibits nucleic acid synthesis


Antibiotics that inhibit protein synthesis by targeting the ribosome ?

-Macrolides such as erythromycin, clarithromycin and azithromycin
-Tetracyclines such as doxycycline



When taking an allergy history always ask what reaction pts have with that medication. If they report diarrhoea for example, this is a side effect rather than an allergy and means if necessary (for example in life threatening sepsis) they can still receive that medication


Stepwise process of escalating antibiotic treatment ?

Start with AMOXICILLIN which covers streptococcus, listeria and enterococcus

Switch to CO-AMOXICLAV to additionally cover staphylococcus, haemophilus and e. coli

Switch to TAZOCIN to additionally cover pseudomonas

Switch to MEROPENEM to additionally cover ESBLs


Add CLARITHROMYCIN or DOXYCYCLINE to cover atypical bacteria


What is sepsis ?

A condition where the body launches a large immune response to an infection that causes systemic inflammation and affects organ function


Pathophysiology of sepsis ?

Pathogens are recognised by macrophages, lymphocytes and mast cells. These cells release vast amounts of cytokines like interleukins and tumour necrosis factor to alert the immune system to the invader. These cytokines activate other parts of the immune system. This immune activation leads to further release of chemicals such as nitrous oxide that causes vasodilation. The immune response causes inflammation throughout the body.

Many of these cytokines cause the endothelial lining of blood vessels to become more permeable. This causes fluid to leak out of the blood into the extracellular space, leading to oedema and a reduction in intravascular volume. The oedema around blood vessels creates a space between the blood and the tissues, reducing the amount of oxygen that reaches the tissues.

Activation of the coagulation system leads to deposition of fibrin throughout the circulation, further compromising organ and tissue perfusion. It also leads to consumption of platelets and clotting factors as they are being used up to form the clots. This leads to thrombocytopenia, haemorrhages and an inability to form clots and stop bleeding. This is called disseminated intravascular coagulopathy (DIC).

Blood lactate rises due to anaerobic respiration in the hypo-perfused tissues with inadequate oxygen. A waste product of anaerobic respiration is lactate.


What is septic shock ?

It is defined when arterial blood pressure drops resulting in organ hypo-perfusion. This leads to a rise in blood lactate as the organs begin anaerobic respiration. This can be measured as either:
-Systolic blood pressure less than 90 despite fluid resuscitation
-Hyperlactaemia (lactate > 4 mmol/L)


Management of septic shock ?

Should be treated aggressively with IV fluids to improve the BP and tissue perfusion. If IV fluid boluses don't improve the BP and lactate level the pt should be escalated to the HDU or ICU where they can use inotropes (such as noradrenaline) that help stimulate the CVS and improve BP and tissue perfusion.


What is severe sepsis ?

It is defined as when sepsis is present and results in organ dysfunction, for example:
-Coagulation dysfunction
-Hyperlactaemia (>2 mmol/L)