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M. B. Vainshtein and E. B. Kudryashova
Skryabin Institute of Biochemistry and Physiology of Microorganisms, Russian Academy of Sciences, pr. Nauki 5, Pushchino, Moscow oblast, 142292 Russia Received April 20, 1999 Abstract: Bacterial nannocells 0.2-0.3 ~tm in size and hundredths of a cubic ~tm in volume have been revealed in natural habitats and obtained in pure cultures. Thetaxonomic analysis of naturally occurring nannobacteria showed that they belong to the known taxa of the kingdom Eubacteria. The results of the cytological investigation of nannocells suggest that they are universally formed in response to stress impacts. Microbiologists usually deal with bacterial cells 2-5 lam in size and several tenths of a cubic in volume, while smaller bacterial forms aremuch less frequently encountered. Some of the reasons for this are the low resolution of light microscopy (about 0.2 lam) [1] and the invisibility of the ultramicrocolonies produced by bacterial miniforms on rich nutrient media to the naked eye [2]. Thus, methodological difficulties in investigating bacterial miniforms have created the impression that they are rare in nature. To emphasize thescarcity of bacterial miniforms, researchers called them ultramicrobacteria or nannobacteria. The latter term implies that the dimensions of nannobacteria can be more conveniently expressed in nanometers than in micrometers (the prefix nanno- is common to biology, while nano-, to physics [3]). Tiny cells formed in laboratory cultures from normal cells about 1 [am in size are also called elementarybodies, subunits, or filterable forms (for details, see below). The older term minicells refers to small cells formed from normal cells due to their division or budding; such cells do not contain hereditary material in amounts sufficient for growth and/or reproduction [4, 5] and will not be considered here. It is generally accepted that the minimum size of viable prokaryotic cells is close to 0.2 lam[1, 7], although some authors believe that the minimum size of bacterial cells is less than 0.1 ~tm [6]. Theoretically, viable bacterial cells may be 0.14 ~m in diameter [8, 9]. The interest of researchers in bacterial miniforms grew as new important findings were reported, e.g., the discovery of nannocells inside Koch's bacilli [10, 11], elementary bodies in the bacterial cells treated withantibiotics [12-14], and filterable bacterial forms as causative agents of infectious diseases [15, 16]. Among recent findings, noteworthy is the detection of growing nannobacteria in the blood serum [ 17, 18] and of nannofossils in Martian meteorites [19-21]. In our recent publication [22], we described the formation of nannocells and nannoforms (0.1-0.6 lam) in pure bacterial cultures initiallycontaining "normal" cells 2-5 lttm in size. The present review is an attempt to analyze relevant data available in the literature and our own experimental data on the nannoforms of bacterial cells. PROBLEMS ASSOCIATED WITH COUNTING VIABLE BACTERIAL MINIFORMS Exact enumeration of bacterial nannocells in mixtures of L-forms or in natural microflora by transmission electron microscopy presents considerabledifficulties related to the process of specimen preparation. In particular, to prevent contamination of the specimen surface with organic compounds and salt crystals, excess liquid should be removed with a piece of filter paper. This may lead to the leakage of liquid, together with small suspended particles, from under a cover glass and, hence, to the removal of some of the nannobacteria from thespecimen. For this reason, scanning electron microscopy reveals much more nannobacteria than transmission electron microscopy. After

detecting a great number of nannobacteria on the surface of geological samples, some authors even claimed that bacteria with normal sizes comprise only a small part of the natural microflora [23-30]. This conclusion is in agreement with the data of 16S rRNA...
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