What is the difference between identification and characterization of a bacterium?

Identification names the organism to genus and species; characterization goes further to determine traits or to type the strain - for example to tell whether isolates from different patients belong to the same outbreak.

Why has mass spectrometry become common for bacterial identification?

MALDI-TOF mass spectrometry can identify many bacteria to species level within minutes of having an isolate by matching the organism's protein mass spectrum to a reference database, which made routine identification faster than many traditional biochemical panels (Clark et al., 2013).

Bacterial Identification and Characterization

Bacterial identification and characterization is the set of laboratory methods used to recover a bacterium from a clinical specimen and determine what it is - its genus and species, and where relevant its strain - so that the result can support diagnosis, treatment selection, and surveillance. Modern laboratories combine classical growth-based and biochemical methods with proteomic identification by mass spectrometry and with nucleic-acid sequencing.

Definition

Bacterial identification is the laboratory process of determining the genus and species (and, where needed, the strain or type) of a bacterium isolated from or detected in a clinical specimen, using phenotypic, proteomic, and genotypic methods.

Scope

The entry covers primary isolation and microscopy, phenotypic identification by growth and biochemical reactions, proteomic identification by MALDI-TOF mass spectrometry, and genotypic identification by gene sequencing, together with the distinction between identification (naming the organism) and characterization (typing and trait determination). It treats these as laboratory methods rather than as clinical guidance.

Core questions

Which bacterial species is present in this specimen, and is its recovery clinically meaningful?
Which identification method - culture and biochemistry, MALDI-TOF mass spectrometry, or gene sequencing - is appropriate for this organism?
How are organisms characterised beyond species level, for example by typing for outbreak investigation?
What are the limits of each method in speed, accuracy, and ability to resolve closely related species?

Key concepts

Primary isolation and pure culture
Gram stain and colonial morphology
Biochemical and phenotypic profiling
MALDI-TOF mass spectrometry identification
16S rRNA and other gene sequencing
Identification versus characterization (typing)
Reference database and spectral matching
Turnaround time

Mechanisms

Conventional identification begins with isolating a pure culture, then reading growth on selective and differential media, Gram-stain morphology, and a panel of biochemical reactions to assign the organism to a species. Proteomic identification by MALDI-TOF mass spectrometry instead ionises proteins from a colony and matches the resulting mass spectrum - dominated by abundant ribosomal proteins - against a reference database, yielding a species-level identification in minutes once an isolate is available; this approach substantially changed routine laboratory workflows (Clark et al., 2013). Genotypic identification sequences conserved genes such as 16S rRNA or uses broader nucleic-acid methods, allowing identification directly from specimens or of organisms that grow poorly (Espy et al., 2006). Each route trades off speed, cost, and discriminatory power, and faster identification has been linked to the broader push for improved infectious-disease diagnostics (Caliendo et al., 2013).

Clinical relevance

An accurate organism identification is a starting point for clinical reasoning about an infection and for choosing which susceptibility tests to perform. This entry describes how identification is achieved and what limits it; it is reference material and does not prescribe how a particular result should change the care of an individual patient.

Epidemiology

Beyond naming an organism, characterization and typing methods underpin outbreak investigation and surveillance by determining whether isolates from different patients are related. The shift to mass spectrometry and molecular methods has been part of a wider effort to make pathogen detection faster and more informative (Caliendo et al., 2013).

History

Bacterial identification rested for most of the twentieth century on culture, staining, and biochemical phenotyping. The introduction of nucleic-acid sequencing and then of MALDI-TOF mass spectrometry into routine laboratories changed both the speed and the breadth of identification, with mass spectrometry in particular described as a fundamental shift in routine practice (Clark et al., 2013) and molecular amplification expanding what could be detected directly from specimens (Espy et al., 2006).

Seminal works

clark-2013
espy-2006
caliendo-2013

Frequently asked questions

What is the difference between identification and characterization of a bacterium?: Identification names the organism to genus and species; characterization goes further to determine traits or to type the strain - for example to tell whether isolates from different patients belong to the same outbreak.
Why has mass spectrometry become common for bacterial identification?: MALDI-TOF mass spectrometry can identify many bacteria to species level within minutes of having an isolate by matching the organism's protein mass spectrum to a reference database, which made routine identification faster than many traditional biochemical panels (Clark et al., 2013).