Human isolates of Listeria monocytogenes in Sweden during half a century (1958-2010).

Isolates of Listeria monocytogenes (n = 932) isolated in Sweden during 1958-2010 from human patients with invasive listeriosis were characterized by serotyping and pulsed-field gel electrophoresis (PFGE) (AscI). Of the 932 isolates, 183 different PFGE types were identified, of which 83 were each represented by only one isolate. In all, 483 serovar 1/2a isolates were distributed over 114 PFGE types; 90 serovar 1/2b isolates gave 32 PFGE types; 21 serovar 1/2c isolates gave nine PFGE types; three serovar 3b isolates gave one PFGE type; and, 335 serovar 4b isolates gave 31 PFGE types. During the 1980s in Sweden, several serovar 4b cases were associated with the consumption of European raw soft cheese. However, as cheese-production hygiene has improved, the number of 4b cases has decreased. Since 1996, serovar 1/2a has been the dominant L. monocytogenes serovar in human listeriosis in Sweden. Therefore, based on current serovars and PFGE types, an association between human cases of listeriosis and the consumption of vacuum-packed gravad and cold-smoked salmon is suggested.

Rapid high-performance liquid chromatographic method for determination of beta-carotene in milk.

An HPLC method for the determination of beta-carotene in milk samples is described. The method has many advantages over previous methods due to its rapidity, convenience and the small volumes of solvents required. Beta-Carotene was 100% recovered and the relative standard deviation of repeatability was 5.3%. The limit of quantitation was found to be about 30 ng/ml and the system was linear in the concentration range investigated, from 0.67 to 4 microgram/ml.

More than one variant of Listeria monocytogenes isolated from each of two human cases of invasive listeriosis.

Two variants of Listeria monocytogenes were isolated from blood cultures from each of two patients with listeriosis. Each variant displayed a two-band difference in DNA profile from the other by pulsed-field gel electrophoresis. Although this difference in profile is insufficient to distinguish clearly between the variants, the possibility of co-infection with different strains of L. monocytogenes needs to be considered. We suggest that more than one colony should be selected for molecular typing to aid interpretation during investigation of the sources and routes of Listeria infection.

Causes behind a human cheese-borne outbreak of gastrointestinal listeriosis.

In a previous paper, we reported an outbreak of gastrointestinal listeriosis due to consumption of fresh cheese made from raw milk and manufactured on a summer farm. The aim of the present study was to investigate why the cheese harbored Listeria monocytogenes. To our knowledge, this is the first documented outbreak of listeriosis caused by raw milk cheese where the human epidemic strain has been cultured from a dairy animal, whose milk has been used for cheese production. The conditions on a summer farm can hardly fulfil the requirements for hygienic and strictly controlled conditions necessary for safe processing of fresh cheese.

A listeriosis patient infected with two different Listeria monocytogenes strains.

Normally, only one isolate of Listeria monocytogenes from a case of listeriosis is subjected to characterization. Here we show that two isolates from different sites of the body were not the same strain. Such a phenomenon may not have any clinical relevance, although it may confuse the epidemiologist trying to match infection source with infection target.