Stroke and Vascular Territories

Taken from Paul Bolin's youtube channel on tutorial of stroke syndromes: https://youtu.be/NfST1Vq8skI

Intracranial Hemorrhage CT

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taken from: https://www.ebmedicine.net/topics.php?paction=showTopicSeg&topic_id=117&seg_id=2285

Bacterial AGE

Short notes on microbiology: Salmonella

Adapted from: http://textbookofbacteriology.net/ken_todar.html

In humans, Salmonella are the cause of two diseases called salmonellosis: enteric fever (typhoid), resulting from bacterial invasion of the bloodstream, and acute gastroenteritis, resulting from a foodborne infection/intoxication.


Figure 1. Salmonella typhi, the agent of typhoid. Gram stain. (CDC)

The genus Salmonella is a member of the family Enterobacteriaceae

Antigenic Structure

As with all Enterobacteriaceae, the genus Salmonella has three kinds of major antigens with diagnostic or identifying applications: somatic, surface, and flagellar.

Somatic (O) or Cell Wall Antigens
Somatic antigens are heat stable and alcohol resistant. Cross-absorption studies individualize a large number of antigenic factors, 67 of which are used for serological identification. O factors labeled with the same number are closely related, although not always antigenically identical.

Surface (Envelope) Antigens
Surface antigens, commonly observed in other genera of enteric bacteria (e.g.,Escherichia coli and Klebsiella), may be found in some Salmonella serovars. Surface antigens in Salmonella may mask O antigens, and the bacteria will not be agglutinated with O antisera. One specific surface antigen is well known: the Vi antigen. The Vi antigen occurs in only three Salmonella serovars (out of about 2,200): Typhi, Paratyphi C, and Dublin. Strains of these three serovars may or may not have the Vi antigen.

Flagellar (H) Antigens
Flagellar antigens are heat-labile proteins. Mixing salmonella cells with flagella-specific antisera  gives a characteristic pattern of agglutination (bacteria are loosely attached to each other by their flagella and can be dissociated by shaking). Also, antiflagellar antibodies can immobilize bacteria with corresponding H antigens.

A few Salmonella entericaserovars (e.g., Enteritidis, Typhi) produce flagella which always have the same antigenic specificity.  Such an H antigen is then called monophasic. Most Salmonella serovars, however, can alternatively produce flagella with two different H antigenic specificities. The H antigen is then called diphasic. For example, Typhimurium cells can produce flagella with either antigen i or antigen 1,2. If a clone is derived from a bacterial cell with H antigen i, it will consist of bacteria with i flagellar antigen. However, at a frequency of 10^-3- 10^-5, bacterial cells with 1,2 flagellar antigen pattern will appear in this clone.

Figure 2. Flagellar stain of a Salmonella Typhi. Like E. coli, Salmonella are motile by means of peritrichous flagella. A close relative that causes enteric infections is the bacterium Shigella. Shigella is not motile, and therefore it can be differentiated fromSalmonella on the bais of a motility test or a flagellar stain. (CDC)

The principal habitat of the salmonellae is the intestinal tract of humans and animals. Typhi and Paratyphi A  are strictly human serovars that may cause  grave diseases often associated with invasion of the bloodstream. Salmonellosis in these cases is transmitted through fecal contamination of water or food. 

Ubiquitous (non-host-adapted) Salmonella serovars (e.g., Typhimurium) cause very diverse clinical symptoms, from asymptomatic infection to serious typhoid-like syndromes in infants or certain highly susceptible animals (mice). In human adults, ubiquitous Salmonella organisms are mostly responsible for foodborne toxic infections.

Salmonella in the Natural Environment
Salmonellae are disseminated in the natural environment (water, soil, sometimes plants used as food) through human or animal excretion. Humans and animals (either wild or domesticated) can excrete Salmonella either when clinically diseased or after having had salmonellosis, if they remain carriers. Salmonella organisms do not seem to multiply significantly in the natural environment (out of digestive tracts), but they can survive several weeks in water and several years in soil if conditions of temperature, humidity, and pH are favorable.

Pathogenesis of Salmomella Infections in Humans

An oral dose of at least 10⁵Salmonella Typhi cells are needed to cause typhoid in 50% of human volunteers, whereas at least 10⁹ S. Typhimurium cells (oral dose) are needed to cause symptoms of a toxic infection. Infants, immunosuppressed patients, and those affected with blood disease are more susceptible to Salmonella infection than healthy adults.

In the pathogenesis of typhoid the bacteria enter the human digestive tract, penetrate the intestinal mucosa (causing no lesion), and are stopped in the mesenteric lymph nodes. There, bacterial multiplication occurs, and part of the bacterial population lyses. From the mesenteric lymph nodes, viable bacteria and LPS (endotoxin) may be released into the bloodstream resulting in septicemia  Release of endotoxin is responsible for cardiovascular �collapsus and tuphos� (a stuporous state�origin of the name typhoid) due to action on the ventriculus neurovegetative centers.

Salmonella excretion by human patients may continue long after clinical cure. Asymptomatic carriers are potentially dangerous when unnoticed. About 5% of patients clinically cured from typhoid remain carriers for months or even years. Antibiotics are usually ineffective on Salmonella carriage (even if salmonellae are susceptible to them) because the site of carriage may not allow penetration by the antibiotic.

Salmonellae survive sewage treatments if suitable germicides are not used in sewage processing. In a typical cycle of typhoid, sewage from a community is directed to a sewage plant. Effluent from the sewage plant passes into a coastal river where edible shellfish (mussels, oysters) live. Shellfish concentrate bacteria as they filter several liters of water per hour. Ingestion by humans of these seafoods (uncooked or superficially cooked) may cause typhoid or other salmonellosis. Salmonellae do not colonize or multiply in contaminated shellfish.

Typhoid is strictly a human disease.The incidence of human disease decreases when the level of development of a country increases (i.e., controlled water sewage systems, pasteurization of milk and dairy products). Where these hygienic conditions are missing, the probability of fecal contamination of water and food remains high and so is the incidence of typhoid.

Foodborne Salmonella toxic infections are caused by ubiquitous Salmonellaserovars (e.g., Typhimurium). About 12-24 hours following ingestion of contaminated food (containing a sufficient number of Salmonella), symptoms appear (diarrhea, vomiting, fever) and last 2-5 days. Spontaneous cure usually occurs.

Salmonella may be associated with all kinds of food. Contamination of meat (cattle, pigs, goats, chicken, etc.) may originate from animal salmonellosis, but most often it results from contamination of muscles with the intestinal contents during evisceration of animals, washing, and transportation of carcasses. Surface contamination of meat is usually of little consequence, as proper cooking will sterilize it (although handling of contaminated meat may result in contamination of hands, tables, kitchenware, towels, other foods, etc.). However, when contaminated meat is ground, multiplication of Salmonella may occur within the ground meat and if cooking is superficial, ingestion of this highly contaminated food may produce aSalmonellainfection. Infection may follow ingestion of any food that supports multiplication of Salmonella such as eggs, cream, mayonnaise, creamed foods, etc.), as a large number of ingested salmonellae are needed to give symptoms. Prevention of Salmonella toxic infection relies on avoiding contamination (improvement of hygiene), preventing multiplication of Salmonella in food (constant storage of food at 4°C), and use of pasteurized and sterilized milk and milk products. Vegetables and fruits may carry Salmonella when contaminated with fertilizers of fecal origin, or when washed with polluted water.

The incidence of foodborne Salmonella infection/toxication remains reletavely high in developed countries because of commercially prepared food or ingredients for food. Any contamination of commercially prepared food will result in a large-scale infection. In underdeveloped countries, foodborne Salmonella intoxications are less spectacular because of the smaller number of individuals simultaneously infected, but also because the bacteriological diagnosis of Salmonella toxic infection may not be available. However, the incidence of Salmonella carriage in underdeveloped countries is known to be high.

Salmonella epidemics may occur among infants in pediatric wards. The frequency and gravity of these epidemics are affected by hygienic conditions, malnutrition, and the excessive use of antibiotics that select for multiresistant strains.

Salmonella Enteritidis Infection
Egg-associated salmonellosis is an important public health problem in the United States and several European countries. Salmonella  Enteritidis, can be inside perfectly normal-appearing eggs, and if the eggs are eaten raw or undercooked, the bacterium can cause illness. During the 1980s, illness related to contaminated eggs occurred mosy frequently in the northeastern United States, but now illness caused by S. Enteritidis is increasing in other parts of the country as well.

Unlike eggborne salmonellosis of past decades, the current epidemic is due to intact and disinfected grade A eggs. Salmonella Enteritidis silently infects the ovaries of healthy appearing hens and contaminates the eggs before the shells are formed. Most types of Salmonella live in the intestinal tracts of animals and birds and are transmitted to humans by contaminated foods of animal origin. Stringent procedures for cleaning and inspecting eggs were implemented in the 1970s and have made salmonellosis caused by external fecal contamination of egg shells extremely rare. However, unlike eggborne salmonellosis of past decades, the current epidemic is due to intact and disinfected grade A eggs. The reason for this is that SalmonellaEnteritidis silently infects the ovaries of hens and contaminates the eggs before the shells are formed.

Although most infected hens have been found in the northeastern United States, the infection also occurs in hens in other areas of the country. In the Northeast, approximately one in 10,000 eggs may be internally contaminated. In other parts of the United States, contaminated eggs appear less common. Only a small number of hens seem to be infected at any given time, and an infected hen can lay many normal eggs while only occasionally laying an egg contaminated with  Salmonella  Enteritidis.

A person infected with the Salmonella  Enteritidis usually has fever, abdominal cramps, and diarrhea beginning 12 to 72 hours after consuming a contaminated food or beverage. The illness usually lasts 4 to 7 days, and most persons recover without antibiotic treatment. However, the diarrhea can be severe, and the person may be ill enough to require hospitalization.  The elderly, infants, and those with impaired immune systems (including HIV) may have a more severe illness. In these patients, the infection may spread from the intestines to the bloodstream, and then to other body sites and can cause death unless the person is treated promptly with antibiotics.

Isolation and Identification

Salmonella sp.

Colony morphology:

•On MacConkey agar, Salmonellas sp. appears as small, round, flat and colourless colonies.

•Absence of pink colour indicates that Salmonellas sp. is a non lactose-fermenter.

Deoxycholate Citrate Agar (DCA)

•Type: Selective medium

•Usage: solid medium used for tha isolation of enteric pathogens, especially Salmonella and Shigella

•Utilise lactose as the only fermentable carbohydrate and employ a neutral red as an indicator

•Ferric ammonium citrate and sodium thiosulphate as H2S indicator (black-centered colony)

Table 1. Characteristics shared by most Salmonella strains belonging to subspecies I 

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Motile, Gram-negative bacteria 
Lactose negative; acid and gas from glucose, mannitol, maltose, and sorbitol; no Acid from adonitol, sucrose, salicin, lactose 
ONPG test negative (lactose negative) 
Indole test negative 
Methyl red test positive 
Voges-Proskauer test negative 
Citrate positive (growth on Simmon's citrate agar) 
Lysine decarboxylase positive 
Urease negative 
Ornithine decarboxylase positive 
H₂S produced from thiosulfate 
Do not grow with KCN 
Phenylalanine and tryptophan deaminase negative 
Gelatin hydrolysis negative