12- Microbiology Explains Flashcards
Campylobacter
jejuni
Most common cause of acute infective diarrhoea
Spiral, gram negative rods
Usually infects caecum and terminal ileum. Local
lymphadenopathy is common
May mimic appendicitis as it has marked right iliac
fossa pain
Reactive arthritis is seen in 1-2% of cases
Shigella spp.
Members of the enterobacteriaceae
Gram negative bacilli
Clinically causes dysentery
Shigella soneii is the commonest infective organism
(mild illness)
Usually self limiting, ciprofloxacin may be required if
individual is in a high risk group
Salmonella spp
Facultatively anaerobic, gram negative,
enterobacteriaceae
Infective dose varies according to subtype
Salmonellosis: usually transmitted by infected meat
(especially poultry) and eggs
E. coli
Enteropathogenic
Enteroinvasive: dysentery, large bowel
necrosis/ulcers
Enterotoxigenic: small intestine, travelers diarrhoea
Enterohaemorrhagic: 0157, cause a haemorrhagic
colitis, haemolytic uraemic syndrome and thrombotic
thrombocytopaenic purpura
Yersinia
enterocolitica
Gram negative, coccobacilli
Typically produces a protracted terminal ileitis that
may mimic Crohns disease
Differential diagnosis acute appendicitis
May progress to septicaemia in susceptible
individuals
Usually sensitive to quinolone or tetracyclines
Vibrio cholera
Short, gram negative rods
Transmitted by contaminated water, seafood
Symptoms include sudden onset of effortless
vomiting and profuse watery diarrhoea
Correction of fluid and electrolyte losses are the
mainstay of treatment
Most cases will resolve, antibiotics are not generally
indicated
What is the mechanism of action for quinolones, metronidazole, sulphonamides, and trimethoprim?
Quinolones (e.g. ciprofloxacin), metronidazole, sulphonamides, and trimethoprim inhibit DNA synthesis.
What is the mechanism of action for penicillins and cephalosporins?
Penicillins and cephalosporins inhibit cell wall formation.
What is the mechanism of action for aminoglycosides, chloramphenicol, macrolides, tetracyclines, and fusidic acid?
Aminoglycosides cause misreading of mRNA, chloramphenicol, macrolides (e.g. erythromycin), tetracyclines, and fusidic acid inhibit protein synthesis.
What is the mechanism of action for rifampicin?
Rifampicin inhibits RNA synthesis.
How can humans become infected with Fasciola hepatica?
Humans can become infected with Fasciola hepatica as part of a plant or food trematode infection.
What is Fasciola hepatica commonly known as?
Fasciola hepatica is commonly known as the common liver fluke, a parasitic trematode.
What symptoms are associated with the acute phase of Fasciola hepatica infection?
During the acute phase, the immature worms penetrate the gut, leading to symptoms such as fever, nausea, swollen liver, skin rashes, and extreme abdominal pain.
What is the recommended treatment for Fasciola hepatica infection?
The recommended treatment for Fasciola hepatica infection is triclabendazole.
Some patients may need ERCP
What symptoms are associated with the chronic phase of Fasciola hepatica infection?
During the chronic phase, the mature worms reside in the bile duct and can cause symptoms such as intermittent pain, jaundice, and anemia.
How can Fasciola hepatica be diagnosed?
Fasciola hepatica can be diagnosed through either a stool sample or serology.
What are the common organisms causing surgical infections?
Staphylococcus aureus, Streptococcus pyogenes, Escherichia coli, Campylobacter jejuni, H Polry
What are the characteristics of Staphylococcus aureus?
Gram positive coccus, facultative anaerobe, catalase positive, causes haemolysis on blood agar plates
What are the potential complications caused by Staphylococcus aureus toxins?
Toxic shock syndrome (exotoxin) and gastroenteritis (enterotoxin)
What is the ideal treatment for Staphylococcus aureus?
Penicillin, although resistance through beta-lactamase production is common
What virulence factors does Streptococcus pyogenes release into the host?
Hyaluronidase, streptokinase, and pyogenic exotoxin A
What is the prevalence of penicillin sensitivity in Staphylococcus aureus isolates in the UK?
Less than 5%
What are the characteristics of Streptococcus pyogenes?
Gram positive, forms chain-like colonies, Lancefield Group A Streptococcus, produces beta haemolysis
What condition can be caused by Streptococcus pyogenes releasing pyogenic exotoxin A?
Scarlet fever
Which antibiotics are effective against Streptococcus pyogenes?
Penicillin (first-line) and macrolides (alternative)
What are the characteristics of Escherichia coli?
Gram negative rod, facultative anaerobe, non-sporing
What are the potential complications of Enterotoxigenic E. coli infection?
Large volume fluid secretion into the gut lumen and diarrhea
What are the characteristics of Campylobacter jejuni?
Curved, gram negative, non-sporulating bacteria
What is one of the commonest causes of diarrhea worldwide?
Campylobacter jejuni
What is the recommended approach for treating Campylobacter jejuni infection?
Antibiotics are not usually advised, but quinolones are often effective
What are the characteristics of Helicobacter pylori?
Gram negative, helix shaped rod, microaerophillic, flagellated, and mobile
What does Helicobacter pylori produce that can derive energy from hydrogen released by intestinal bacteria?
Hydrogenase
What can Helicobacter pylori secrete that breaks down gastric urea?
Urease
How can Helicobacter pylori infection be diagnosed?
Serology or biopsy urease test during endoscopy
What is the approximate risk of peptic ulcer in patients colonized by Helicobacter pylori?
What is the approximate risk of gastric cancer in patients colonized by Helicobacter pylori?
What is the approximate risk of MALT lymphoma in patients colonized by Helicobacter pylori?
10-20%
1-2%
Less than 1%
What is necrotizing fasciitis?
Advancing soft tissue infection associated with fascial necrosis
What type of flora is responsible for causing necrotizing fasciitis, and what pathogen is increasingly seen in these cases?
Polymicrobial flora (aerobic and anaerobic), and Methicillin-Resistant Staphylococcus aureus (MRSA) is seen more frequently
Which organism is the most common isolated pathogen in necrotizing fasciitis infections?
Streptococcus (15% of cases)
What is Meleney’s gangrene?
A similar principle to necrotizing fasciitis, but the infection is more superficially located and often confined to the trunk
What is Fournier’s gangrene?
Necrotizing fasciitis affecting the perineum
What are the clinical features of necrotizing fasciitis?
Fever, pain, cellulitis, edema, induration, numbness; late findings include purple/black skin discoloration, blistering, hemorrhagic bullae, crepitus, and dirty dishwater fluid discharge
What types of bacteria are commonly involved in Fournier’s gangrene?
Polymicrobial infection with E. coli and Bacteroides acting synergistically
What is the cornerstone of management for necrotizing fasciitis?
Radical surgical debridement
What is the recommended wound care for necrotizing fasciitis?
Sterile dressing
What is the causative agent of venereal syphilis?
Treponema pallidum (a spirochete)
What other mode of transmission can lead to syphilis?
Congenital infection
What is the typical lesion seen in primary syphilis?
Chancre (an indurated papule that forms an ulcer)
How long does it take for the chancre to appear after infection?
10-90 days
How does the chancre typically feel and how does it mimic carcinoma?
Hard and painless; it can resemble a hard ulcer with enlarged regional lymph nodes
What are the histological appearances seen on biopsy of the chancre?
Non-specific dense inflammatory infiltrates
How can the diagnosis of syphilis be confirmed?
By demonstrating spirochetes from the chancre using dark ground illumination microscopy or through serological testing for anti-treponemal antibodies
When does secondary syphilis typically develop?
Around 6 weeks following exposure to the disease
What is a common manifestation of secondary syphilis?
Widespread roseolar eruption
What can happen after treatment of syphilis?
Jarisch-Herxheimer reaction, characterized by fever and malaise, due to the release of antigens as the organisms die
What can occur in individuals who are not treated for syphilis?
Development of tertiary syphilis, characterized by the formation of gummas or diffuse inflammatory lesions
Which part of the aorta does syphilis typically affect in tertiary syphilis?
Proximal aspect
What can result from syphilis affecting the aorta?
Atypical aneurysms
What percentage of healthcare-associated infections do SSI comprise?
Up to 20%
What can cause surgical site infections (SSI)?
Breach in tissue surfaces that allow normal commensals and other pathogens to initiate infection
What percentage of patients undergoing surgery will develop an SSI?
At least 5%
Where do the organisms causing SSI often come from?
The patient’s own body
What are some measures that may increase the risk of SSI?
Shaving the wound with a razor (preferably use a disposable clipper), using a non-iodine impregnated incise drape, tissue hypoxia, delayed administration of prophylactic antibiotics in tourniquet surgery
When should body hair be removed prior to surgery?
It should not be removed routinely; if necessary, use electrical clippers with a single-use head (razors increase infection risk)
When should antibiotic prophylaxis be considered?
For placement of prosthesis or valve, clean-contaminated surgery, and contaminated surgery; use antibiotics from the local formulary and aim to give a single dose intravenous antibiotic during anesthesia
What should be used to prepare the skin before surgery for the lowest incidence of SSI?
Alcoholic chlorhexidine
What should be done to cover the surgical site during the procedure?
Apply a dressing
Does administration of supplementary oxygen reduce the risk of wound infection?
No, recent meta-analysis has confirmed that it does not reduce the risk of wound infection
Do wound edge protectors confer benefit in preventing SSI?
No, they do not appear to confer benefit
What tissue viability advice should be followed for management of surgical wounds healing by secondary intention?
Follow tissue viability advice
Is the use of diathermy for skin incisions advocated in the NICE guidelines?
No, it is not advocated
Has the use of diathermy for skin incisions been shown to increase the risk of SSI?
No, several randomized controlled trials have demonstrated no increase in risk of SSI when diathermy is used
What organism causes enterobiasis?
Enterobius vermicularis
What is the common symptom of enterobiasis?
Pruritus ani (itching around the anus)
How is the diagnosis of enterobiasis usually made?
By placing scotch tape at the anus to trap eggs for microscopic examination
What are the common symptoms of Ancylostoma duodenale infection?
Most infections are asymptomatic, but it can cause iron deficiency anemia
What is the treatment for enterobiasis?
Mebendazole
How can Ancylostoma duodenale infection be diagnosed?
Larvae may be found in stools left at ambient temperature; otherwise, infection is difficult to diagnose
What is the treatment for Ancylostoma duodenale infection?
Mebendazole
How does Ancylostoma duodenale infection occur?
Infection occurs through cutaneous penetration, migration to lungs, being coughed up, and then swallowed
Which parasite causes Ascariasis?
Ascaris lumbricoides (roundworm)
How does Ascariasis infection occur?
Infections begin in the gut following ingestion, then the worms penetrate the duodenal wall to migrate to the lungs, are coughed up, and then swallowed
How is the diagnosis of Ascariasis made?
By identifying worms or eggs in the feces
What is the treatment for Ascariasis?
Mebendazole
Which parasite causes Strongyloidiasis?
Strongyloides stercoralis
How does Strongyloidiasis infection occur?
Initial infection is via skin penetration, then migration to the lungs, being coughed up and swallowed; mature worms in the small bowel are excreted, and the cycle begins again
What are the symptoms of Strongyloidiasis?
Individuals may be asymptomatic, but can also have respiratory disease and skin lesions
What is the autoinfective cycle recognized in Strongyloidiasis?
Larvae penetrating the colonic wall
How is the diagnosis of Strongyloidiasis usually made?
By stool microscopy
What is the treatment for Strongyloidiasis in the UK?
Mebendazole
What is the causative organism of Cryptosporidium infection?
Cryptosporidium (a protozoan)
How is Cryptosporidium infection transmitted?
Through the excretion of cysts, causing new infections
What are the symptoms of Cryptosporidium infection?
Diarrhea and cramping abdominal pains, which are worse in immunosuppressed people
How can Cryptosporidium infection be diagnosed?
Identification of cysts in stools
What is the treatment for Cryptosporidium infection?
Metronidazole