ABSTRACT

Objective: Virulent strains of the Mycobacterium tuberculosis complex, under certain appropriate conditions, grow as characteristic ropes, bundles or serpentine cords known as cord factor or growth in cords. The objective of the present study was to evaluate cord factor detection as a method of achieving presumptive identification of the M. tuberculosis complex, comparing it to conventional typing tests. Methods: A total of 743 strains were analyzed from January of 2002 to December of 2005 in the Mycobacteria Sector of the Adolfo Lutz Institute, located in the city of Santos, Brazil. Samples were obtained from clinical specimens collected from patients with respiratory symptoms treated at basic health clinics in the greater metropolitan area of Santos. Ziehl-Neelsen-stained smears were prepared, 301 (40.5%) in MB/BacT broth and 442 (59.5%) on solid media, either Lowenstein-Jensen or Ogawa-Kudoh. Results: The sensitivity, specificity, positive predictive value and negative predictive value obtained during the performance comparison of the two methods (cord factor detection and conventional typing) using both isolation media were, respectively, 98.5, 88, 97 and 93%. The method was more sensitive on solid medium (100%), and the difference in sensitivity between the two media types was only 2.7%. Conclusions: Taking into consideration the results obtained, we conclude that, in laboratories with a high incidence of M. tuberculosis complex isolation and limited economic resources, cord factor detection is a fast and valid criterion for identifying these mycobacteria using liquid or solid medium. It also enables subsequent conclusive identification tests, as well as additional sensitivity tests when necessary.

Keywords: Laboratory techniques and procedures; Mycobacterium tuberculosis; Cord factors

RESUMO

Objetivo: O Mycobacterium tuberculosis, sob certas condições apropriadas, cresce em cordões de serpentinas, denominados de fator corda, ou crescimento em cordas. O objetivo deste estudo é avaliar a detecção do fator corda como método de identificação presuntiva do complexo M. tuberculosis, comparando-o aos testes de tipificação (TIP) convencionais. Método: Foram analisadas 743 cepas, de janeiro de 2002 a dezembro de 2005, na Área de Micobactérias do Instituto Adolfo Lutz - Santos, obtidas de isolados clínicos coletados de pacientes sintomáticos respiratórios ou com suspeita clínica de tuberculose pulmonar e/ou micobacterioses, atendidos nas Unidades Básicas de Saúde da Baixada Santista. Foram feitos esfregaços das cepas de micobactérias isoladas em meio líquido MB/BacT e meio sólido, Lowenstein-Jensen ou Ogawa-Kudoh, sendo 301 (40,5%) cepas em meio líquido e 442 (59,5%) em meio sólido. Resultados: Os resultados de sensibilidade, especificidade e valores preditivos positivos e negativos, obtidos com a comparação do desempenho do método em ambos os meios de isolamento e TIP convencionais, foram respectivamente 98,5, 88, 97 e 93%. Observou-se maior sensibilidade do método em meio sólido (100%), com uma diferença de sensibilidade entre os meios analisados de apenas 2,7%. Conclusões: Conclui-se, pelos resultados obtidos, que o fator corda é um critério real e rápido na identificação do complexo M. tuberculosis; além disso, em laboratórios com alta prevalência do complexo M. tuberculosis e que não dispõem de técnicas que permitam a precocidade de sua identificação, o fator corda possibilita o direcionamento aos testes conclusivos de identificação e adicionais de sensibilidade que se façam necessários.

Palavras-chave: Técnicas e procedimentos de laboratório; Mycobacterium tuberculosis; Fatores Cord.

Introduction

Tuberculosis (TB) is one of the most ancient diseases, with wide geographic distribution, constituting a serious public health problem worldwide and in Brazil.(1) It occurs in developed countries, as well as in countries with emerging economies that present deep developing contrasts, and is associated with high poverty indices, making it a public health priority in developing countries.(2)

The etiologic agent of TB is Mycobacterium tuberculosis, and its most common clinical presentation is pulmonary. The most common form of transmission is the person-to-person contact from an infection source (infected individual) with pulmonary lesions.(3)

The World Health Organization (WHO) estimates that TB accounts for 2.7 million deaths annually, 95% of which occur in developing countries. Projections made in 1995 indicate that, by the year 2005, 11.9 million new cases of the disease will occur annually. Brazil is ranked 16th among the countries that account for 80% of the total cases of TB worldwide,(4,5) and presents the greatest number of cases in Latin America.(6) In addition, it is among the 22 countries that the WHO considers priorities,(7) with an incidence rate of 60 per 100,000 inhabitants and a mortality rate of 7.8 per 100,000 inhabitants.(5)

The state of São Paulo reports approximately 21,000 cases annually, which is, in absolute numbers, the highest of all states in Brazil. In 2005, the incidence of TB in the state of São Paulo was 43.9 cases per 100,000 inhabitants. Although it was not the highest in Brazil, it was close to the national average which was, in 2004, 44.1 per 100,000 inhabitants. The state of São Paulo comprises 645 cities, and 53% of the new cases are concentrated in 10 of those cities.(8)

The greater metropolitan area of Santos, a coastal city located in the state of São Paulo, is the third most populated region in the state, with approximately 1,500,000 people distributed in 9 cities. This region comprises 8 of the 73 cities considered priorities for the Programa Nacional de Controle da Tuberculose (PNCT, National Tuberculosis Control Program). In 2002, 2003 and 2004, the incidence of TB in the greater metropolitan area of Santos was 95.2, 99.2 and 94.9 per 100,000 inhabitants, respectively.(8,9) In view of the current epidemiological situation of this disease, the rapid diagnosis of mycobacterioses is a constant challenge for the PNCT.

As a consequence of the glycolipid trehalose dimycolate, present in the bacterial cell wall, and under appropriate conditions, we observe, in the virulent strains of the bacillus of TB, the growth of the M. tuberculosis complex into microscopic serpentine-like cords, denominated cord factor, or growth in cords, in which the acid-fast bacilli (AFB) are in parallel arrays.(10) Studies have evaluated the use of cord factor detection in liquid medium as a reliable presumptive result in the rapid early identification of the M. tuberculosis complex in laboratories that use automated methodology to isolate mycobacteria.(11)

The method has also been used as a triage in the presumptive identification of the M. tuberculosis complex, together with the evaluation of the colony morphology in solid medium, as a practical and low cost pre-test to determine whether additional identification or sensitivity tests are needed.(11,12)

In view of this, and as a Regional Laboratory, we carried out this study with the objective of evaluating the presence of the cord factor as presumptive identification of the M. tuberculosis complex, using mycobacteria strains isolated in a liquid medium (MB/BacT broth) and on a solid medium (Lowenstein-Jensen or Ogawa-Kudoh). We intend to demonstrate that this rapid, easy, sensitive low-cost method can be safely performed in our laboratory and in local laboratories that use the isolation of mycobacteria on solid media.



Methods

This study was conducted based on the analysis of a collection consisting of 743 mycobacteria strains (2002-2005), 301 (40.5%) in liquid medium and 442 (59.5%) on solid medium, isolated from clinical samples collected from patients experiencing respiratory symptoms or clinically suspected of having pulmonary TB or mycobacteriosis and treated at the basic health clinics in the greater metropolitan area of Santos, using techniques recommended by the National Ministry of Health.(13)

Ziehl-Neelsen-stained smears were prepared according to the Guidebook for Tuberculosis Bacteriology(13):

 smeared strain in liquid medium: performed
directly on slide, from the sediment obtained from 5 mL centrifuged in liquid medium.
 smeared strain on solid medium: performed directly on slide with sterile distilled water, from an isolated strain.

Slides were analyzed, and the presence of AFB and cord factor formation was noted.

The identification of the strain as belonging to the M. tuberculosis complex was confirmed by the analysis of the records of conventional typing test results previously performed, in which the strain was submitted to the Gen-Probe test (DNA probes), morphological analysis of growth and other traditional biochemical methods.(14,15)

The study was approved by the Ethics in Human Research Committee of the Adolfo Lutz Institute.

Results

The prevalence of mycobacteria species of the M. tuberculosis complex in the period of the study (2002-2005) was 81%; therefore, presumptive identification of this species is an important diagnostic resource.

Based on the results obtained from the 743strains analyzed, we determined the sensitivity and specificity, as well as positive and negative predictive values, for the presence of the cord factor in the presumptive identification of the M. tuberculosis complex in liquid and on solid medium, in relation to conventional typing tests. To that end, we divided this analysis into three phases: evaluation of cord factor detection in liquid medium; evaluation of cord factor detection on solid medium; and, finally, evaluation of the performance of the method in relation to the total strains analyzed (Table 1).

In the presumptive identification of the M. tuberculosis complex, cord factor detection presented sensitivity, specificity, positive predictive value and negative predictive value of 97.3, 87.6, 96.5 and 90.4%, respectively, in liquid medium, compared with 100, 89, 98 and 100%, respectively, on solid medium.

In Figures 1 and 2, we can observe microscopic images of M. kansasii and M. tuberculosis, respectively, isolated using both types of media.

In the comparison of the performance of the method in both isolation media and in the final identification with conventional typing tests, the sensitivity, specificity, positive predictive value and negative predictive value obtained were, respectively, 98.5, 88, 97 and 93%.



Of the 743 strains studied, 608 (81.8%) were identified as belonging to the M. tuberculosis complex by identifying the cord factor. Of those, 591 (97.2%) were confirmed by conventional typing tests and 17 (2.8%) were identified as nontuberculous mycobacteria (NTM).

The species identified using conventional methods, isolated in liquid and on solid medium, and classified according to the presence or absence of the cord factor, can be seen in Table 2.

Discussion

The sensitivity, specificity, positive predictive value and negative predictive value found in our analysis are comparable to those reported in the literature.(16) Other authors,(17) examining cord factor detection as presumptive result of the M. tuberculosis complex isolated in liquid medium, report 90% sensitivity, similar to that obtained in our study.




It is possible to observe the cord factor in NTM, since they produce 'pseudo cords', that is, incomplete growth in cords, and the interpretation depends on the experience of the laboratory technician (Figure1). Of the total strains analyzed, 17 NTM (12%) were described as cord factor positive, and 53% of those were identified as M. kansasii. These values are considered high when compared with those of other authors.(11,16) However, we emphasize that such studies do not reveal the isolation of M. kansasii. In addition, our values are lower than the 16.9% presented by one well-known author.(12)

Analyzing the performance of the method using both isolation media, only 1.5% of the strains of the M. tuberculosis complex were identified as cord factor negative, 100% of them in liquid medium. Such differences seem insignificant and are lower than the approximately 10% presented by other authors.(12,17)

The sensitivity of the method was greater on solid medium. We found only a 2.7% difference in sensitivity when analyzing the method using both isolation media, a quite relevant number, since most public health laboratories in Brazil use solid medium in the isolation of mycobacteria, confirming the viability of this method.

Statistical calculations to determine concordance values among methods revealed the following: 96% overall concordance, 69% expected concordance and 87% adjusted concordance (kappa).

Based on the positive and negative predictive values obtained in our study, we can conclude that identifying growth in cords is a real and rapid criterion for the identification of the M. tuberculosis complex isolated in liquid or on solid medium, enabling us to refer to conclusive identification tests as well as additional sensitivity tests that are deemed necessary, in laboratories with a high prevalence of M. tuberculosis and in which other techniques for early identification are unavailable.

References

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3. Murray CJL, Stylbo K, Rouillon A. Tuberculosis. In: Jamison DT. Disease control proiorities in developing countries. Oxford: Oxford Medical Publication. Oxford University press; 1993. p. 233-59.

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11. Badak FZ, Goksel S, Sertoz R, Guzelant A, Kizirgil A, Bilgic A. Cord formation in MB/BacT medium is a reliable criterion for presumptive identification of Mycobacterium tuberculosis complex in laboratories with high prevalence of M. tuberculosis. J Clin Microbiol. 1999;37(12):4189-91.

12. Monteiro PHT, Martins MC, Ueki SYM, Giampaglia CMS, Telles MAS. Cord formation and colony morphology for the presumptive identification of Mycobacterium tuberculosis complex. Braz J Microbiol. 2003;34(2):171-4.

13. Fundação Nacional de Saúde. Centro de Referência Professor Helio Fraga. Manual de Bacteriologia da Tuberculose. 2nd ed. Rio de Janeiro: Ministério da Saúde; 1994.

14. Collins CH, Grange JM, Yates MD. Tuberculosis bacteriology: organization and practice. 2nd ed. London: Butterworth-Heinemann; 1997. p. 139.

15. Kent PT, Kubica GP. Public health mycobacteriology y -a guide for level III laboratory. Atlanta: Centers for Disease Control, US Department of Health and Public Services, 1985. p. 207.

16. McCarter YS, Ratkiewicz IN, Robinson A. Cord formation in BACTEC medium is a reliable, rapid method for presumptive identification of Mycobacterium tuberculosis complex. J Clin Microbiol. 1998; 36(9):2769 71.

17. Yagupsky PV, , Kaminski DA, Palmer KM, Nolte FS. Cord formation in BACTEC 7H12 medium for rapid, presumptive identification of Mycobacterium tuberculosis complex. J Clin Microbiol. 1990; 28(6):1451-3.

_______________________________________________________________________________________________________
* Study carried out at the Adolfo Lutz Institute, Santos Regional Laboratory, Santos, Brazil.
1. Assistant Technician in Scientific and Technological Research. Adolfo Lutz Institute, Santos Regional Laboratory, Santos, Brazil.

2. Scientific Researcher. Adolfo Lutz Institute, Santos Regional Laboratory, Santos, Brazil.

3. Support Technician in Scientific and Technological Research. Adolfo Lutz Institute, Santos Regional Laboratory, Santos, Brazil.

4. Pharmacist. Adolfo Lutz Institute, Santos Regional Laboratory, Santos, Brazil.

Correspondence to: Andréa Gobetti Vieira Coelho. Rua Oswaldo Cruz, 67/94, Boqueirão, CEP 11045-101, Santos, SP, Brazil.
Tel 55 13 3232-5112. Fax 55 13 3322-3151.
E-mail: dea_gobetti@hotmail.com
Submitted: 25 January 2007. Accepted, after review: 16 April 2007.