Marine microbiologists on the crime scene!

One might not know that sometimes forensic medicines looks to marine microbiology to solve a crime, especially regarding death by drowning . In fact, to establish where the death exactly occurred, forensic scientists investigate the presence of diatoms in the cadaver.

The diatom test, which basically consists in counting and identifying diatoms found in organs and tissues considered relevant for the investigation purposes, has been often used in the past and it is currently considered the gold standard.

However, the technical reliability, sensitivity and high cost of diatom test has caused considerable controversy.

For this reason scientists are now considering a new technique to support or replace the diatoms test findings: the biolumiscent bacteria test.

Why biolumiscent bacteria?

First of all biolumiscent bacteria do not inhabit the river waters, whereas they are prevalent in the sea, where they are ubiquitous: the presence of these bacteria could give information regarding the environment where the death occured, whereas the diatom test is not always reliable.

Secondly, their isolation and culture are easy and relatively cheap.

Moreover, marine bacteria, including bioluminiscent, are smaller than diatoms so when aspirated with water such bacteria could penetrate capillary vessels in the lungs and enter the blood system, where a portion might survive and predominantly proliferate, eventually suppressing the proliferation of endogenous bacteria from the gastrointestinal tract during the agonal period and/or after death, and of other exogenous bacteria that might enter after death.

The authors of this paper contributed in solving 2 death cases.

The first case was a 63-year-old man found in the shallows of a river, about 230m upstream from the mouth of the river.

The second case was 52-year-old man found dead floating in a car about 4m from shore and about 800m upstream from the mouth of another river.

Blood, urine and tissue samples were screened for drugs,alcohol, diatoms and bacterial analysis. Regarding the lattest, blood samples were diluted and then cultured in different NaCl concentration; after incubation, biolumiscent colonies were visualized in the dark. Plates were then stained with cytochrome oxidase reagent, which causes marine Gram-negative bacteria rods to turn blue, but not most of the enteric bacteria that proliferate during human decomposition.

Numerous biolumiscent and blue colonies appeared in the agar plates at all NaCl concentrations for both of the 2 cadavers. Homologous analysis based on the 16S rRNA gene showed that they were composed of marine bacteria belonging the the family Vibrionaceae.

The presence of numerous marine bacteria from the blood samples suggested that the victims had aspirated marine or brackish water.

These results, together with the autopsy findings and laboratory analysis for drugs and diatoms, supported the results of police investigations, indicating that drowning was the cause of death of the two immersed cadavers.

References: Kakizaki, E., S. Kozawa, et al. (2009). "Bioluminescent bacteria have potential as a marker of drowning in seawater: Two immersed cadavers retrieved near estuaries." Legal Medicine 11(2): 91-96.